Mold for producing glittering cube-corner retroreflective sheeting

ABSTRACT

A mold 79 that includes an array of cube-corner elements 80 that are arranged in the array such that a retroreflective cube-corner sheeting 60 that is formed thereon is capable of glittering when light is incident on the sheeting. The array may be defined by three sets of intersecting grooves 25, 26, 27 where each groove set includes two or more generally parallel grooves. At least one groove in at least one of the sets has faces 22 of adjacent cube-corner elements 30a, 30b arranged such that a dihedral angle  alpha  located between adjacent faces varies along the groove(s) in the set.

TECHNICAL FIELD

This invention pertains to a mold for making a cube-cornerretroreflective sheeting that glitters when exposed to light.

BACKGROUND

Retroreflective sheeting is characterized by its ability to reflectsubstantial quantities of incident light back towards the light source.This unique ability has promoted wide-spread use of retroreflectivesheetings on signs, barricades, traffic cones, clothing, and other itemsthat need to be visible at nighttime. Retroreflective sheeting improvesthe conspicuity of the articles onto which the sheeting is placed,particularly at nighttime.

A very common retroreflective sheeting uses an array of cube-cornerelements to retroreflect light. FIGS. 1 and 2 illustrate an example ofsuch a retroreflective sheeting, noted generally by numeral 10. Thearray of cube-corner elements 12 project from a first or rear side of abody portion 14 that includes a body layer 18 (also referred to in theart as an overlay) and may also include a land layer 16. Light entersthe cube-corner sheeting 10 through the front surface 21; it then passesthrough the body portion 14 and strikes the planar faces 22 of thecube-corner elements 12 to return in the direction from which it came asshown by arrow 23.

FIG. 2 shows the back side of the cube-corner elements 12, where eachcube-comer element 12 is in the shape of a trihedral prism that hasthree exposed planar faces 22. The cube-comer elements 12 in knownarrays are typically defined by three sets of parallel v-shaped grooves25, 26, and 27. Adjacent planar faces 22 on adjacent cube-comer elements12 in each groove form an external dihedral angle (a dihedral angle isthe angle formed by two intersecting planes). This external dihedralangle is constant along each groove in the array. This has been the casefor a variety of previously produced cube-corner arrays (including thosedisclosed in the patents cited in the next paragraph).

The planar faces 22 that define each individual cube-corner element 12generally are substantially perpendicular to one another, as in thecorner of a room. The internal dihedral angle--that is, the anglebetween the faces 22 on each individual cube-corner element in thearray--typically is 90°. This internal angle, however, can deviateslightly from 90° as is known in the art; see U.S. Pat. No. 4,775,219 toAppeldorn et al. Although the apex 24 of each cube-corner element 12 maybe vertically aligned with the center of its base (see, for example,U.S. Pat. No. 3,684,348), the apex also may be offset or canted from thecenter as disclosed in U.S. Pat. No. 4,588,258 to Hoopman. Othercube-corner configurations are disclosed in U.S. Pat. Nos. 5,138,488,4,066,331, 3,923,378, 3,541,606, Re 29, 396.

While known cube-corner retroreflective sheetings come in a variety ofconfigurations that provide very effective nighttime retroreflectivity,and hence very effective nighttime conspicuity, known retroreflectivesheetings generally have had somewhat limited conspicuity under daytimelighting conditions. This is because under daytime conditions theretroreflected light is not easily distinguishable from the surroundingambient light. Thus, other measures have been taken to enhance daytimeconspicuity, including adding fluorescent dyes to the retroreflectivesheeting,--see U.S. Pat. Nos. 5,387,458 and 3,830,682. Or, as disclosedin U.S. Pat. No. 5,272,562 to Coderre, white opaque pigment particleshave been dispersed in the front of the cube-corner elements. Althoughthe presently known techniques are very effective for improving aretroreflective sheeting's daytime conspicuity, they possess thedrawback of requiring the addition of another ingredient namely dye orpigment, to achieve the enhanced conspicuity.

SUMMARY OF THE INVENTION

The present invention provides a new and very different approach toimproving a retroreflective sheeting's daytime conspicuity. Rather thanuse a fluorescent dye or bright pigments, as has been done in the priorart, the present invention enhances conspicuity by providing a mold thatis capable of producing a cube-corner retroreflective sheeting thatglitters when exposed to light. In brief the invention is a mold thatcomprises an array of cube-corner elements that are arranged in thearray such that a retroreflective cube-corner sheeting that is formedthereon is capable of glittering when light is incident thereon.

The terms "glitter", "glitters", or "glittering" are used herein to meana multiplicity of discreet regions of light that appear as distinctpoints of light, each of which may be noticed by the unaided eye of anordinary observer when light is incident on the sheeting, but whichpoints of light disappear or become unnoticeable to the eye of the sameobserver when either the angle of the incident light source to thesheeting, the angle of observation, the sheeting's orientation, or acombination thereof are changed. Some points of light may appear, forexample, violet in color, while others may display orange, green, yellowor any of the other colors of the visible spectrum.

In some embodiments, the glittering effect may be seen from both thefront and back sides of sheeting produced by a mold of the inventionwhen light strikes either the front or the back. The glittering effectis particularly noticeable for such sheetings when viewed undersunlight. The glittering effect may be seen from the front side atobservation angles of -90 degrees to +90 degrees from an incidence angleextending normal (zero degrees) to a flat sample. Sheetings produced bya mold of the invention also can glitter when viewed from the backside-90 degrees to +90 degrees from a normal or zero degree incidence angle.Even if the incidence angle is offset from a line normal to thesheeting, the glittering effect may also be noticeable at all viewingangles. As a sample is rotated 360 degrees, the glitter may be seencontinuously. During the rotation, some points of light disappear butothers appear. This provides a broad range of angles over whichcontinual "blinking" on and off of light from different cube-cornersoccurs, resulting in the phenomenon of glitter. Sheetings formed from amold of the invention may be capable of glittering under essentially allpossible illumination and viewing angles, in all combinations.

The glitter enhances the sheeting's daytime conspicuity, and to someextent may also improve its nighttime conspicuity. Molds of theinvention thus can create glittering retroreflective sheetings withaesthetic appeal and may be useful for producing graphic images such asproduct identifiers. These advantages and others are more fullydescribed below in the detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a prior art retroreflective sheeting 10.

FIG. 2 is a bottom view of the prior art retroreflective sheeting 10illustrated in FIG. 1.

FIG. 3 is an isometric view of a cube-corner element 30 that may be usedin a retroreflective sheeting formed from a mold of the invention.

FIG. 4 is a bottom view of a retroreflective sheeting 60 that may bemade from a mold of the present invention.

FIG. 5 is a sectional view of retroreflective sheeting 60 taken alonglines 5--5 of FIG. 4.

FIG. 6 is a bottom view of retroreflective sheeting 60, illustratingapex and groove intersection heights from a reference plane.

FIG. 7 is a sectional view of retroreflective sheeting 60 taken alonglines 7--7 of FIG. 5.

FIG. 8 is a sectional view of a retroreflective product 61 having a sealfilm 63 secured to the backside of the retroreflective sheeting 60.

FIG. 9 is a front view of retroreflective product 61 illustrating a sealpattern that may be used to produce hermetically-sealed chambers 65(FIG. 8) behind the cube-corner elements 30 (FIG. 8).

FIG. 10 illustrates a safety vest 69 that has glittering retroreflectiveproducts 61 on its outer surface 70.

FIG. 11 is a schematic view of how a glittering retroreflective sheetingcan be made by exposing a retroreflective sheeting 10 to heat and/orpressure in a laminating apparatus 71.

FIG. 12 is a schematic view of an alternative method of exposing aretroreflective sheeting 10 to heat and/or pressure to produce aglittering retroreflective sheeting 60.

FIG. 13 is a top view of a mold 79 according to the present inventionwhich may be used to produce a glittering retroreflective sheeting.

FIG. 14 is a schematic view of a second technique for making aretroreflective sheeting 60 in accordance with the present invention bycasting the sheeting from a mold 79.

FIG. 15 is a front view of an imaged retroreflective sheeting 101 thathas glittering and non-glittering regions 102 and 103, respectively.

FIG. 16a is a side view of an insert 104a that may be used to produce animage in a glittering sheeting.

FIG. 16b is a side view of an insert 104b that may be used to produce animage in a glittering sheeting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the practice of the present invention, a retroreflective sheeting canbe produced that exhibits a glittering effect under daytime lightingconditions as well as under nighttime or retroreflective lightingconditions (although not to as noticeable an extent). The glitteringeffect can provide the resultant sheeting with good daytime brightnessor lightness as measured by a standardized test, ASTM E 1349-90, wherelightness is expressed by the Luminance Factor Y (LFY). Clear, colorlesssheetings of the invention may demonstrate a LFY value of 38 or greater,and even 55 or greater. Of course, LFY values may differ depending onthe color of the glittering sheeting. Further, the LFY value may behigher depending on the degree of texture or pattern present in theglittering sheeting. The measurement geometry imposed by ASTM E 1349-90(0°/45° or 45°/0°) excludes detection of substantial portions of thelightness due to glittering because the glittering sheetings reflectlarge amounts of light at angles that are not detected. Sheetingsproduced from molds of the invention may display at least about 10, andpreferably at least about 50, points of light per square centimeter(cm²) when the sheeting is viewed under direct sunlight. Typically,there are less than about 250 points of light per cm² when viewed underdirect sunlight. The glitter is achieved not through incorporating metalparticles or flakes in a sheeting or a coating as is commonly done inthe glittering art--see, e.g., U.S. Pat. Nos. 5,470,058, 5,362,374,5,276,075, 5,202,180, 3,988,494, 3,987,229, 3,697,070, 3,692,731, and3,010,845--but instead is achieved through an entirely different and newapproach, namely, by orienting cube-corner elements in a new geometricarrangement.

In a preferred embodiment of this new geometric arrangement, at leastone set of parallel grooves in an array of cube-corner elements hasfaces of adjacent cube-corner elements arranged such that the externaldihedral angle formed between the faces varies along at least one groovein the set.

In another preferred embodiment, the external dihedral angle betweenfaces of adjacent cubes varies in all grooves to such an extent that thecubes are randomly tilted across the array. What is meant by "randomlytilted" is that the cubes in the sheeting are tilted in a nonrepeatingpattern relative to a reference plane that can be the front surface ofthe retroreflective sheeting when laid flat. A cube is considered"tilted" when its optical axis is not perpendicular to the referenceplane. The "optical axis" is customarily understood as being theinternal line that extends from the cube apex and forms equal angleswith each cube edge that extends from the apex. In other words, theoptical axis is the line defined by the intersection of three planesthat each bisect one of the three internal dihedral angles formed by thecube-corner element's three planar faces.

All previously known retroreflective sheetings have had the cube-cornerelements arranged in a predetermined repeating pattern throughout thearray.

If a known cube-corner sheeting is thought of as an army that marches incadence in strict formation, a randomly-oriented sheeting would be adrunken army where each cube-corner element represents individualsoldiers that stagger and possibly bump into each other as they march.

FIG. 3 illustrates a cube-corner element 30 that is useful inretroreflective sheetings produced from molds of the invention (60, FIG.4) as well as in sheetings of the prior art (10, FIG. 1). As shown, acube-corner element 30 is a body that has three mutually perpendicularfaces 31a, 31b, and 31c that meet at the element's apex 34. Thecube-corner element's base edges 35 are generally linear and generallylie in a single plane that defines the base plane 36 of the element 30.Cube-corner element 30 also has a central or optical axis 37, which isthe tri-sector of the internal angles defined by lateral faces 31a, 31b,and 31c. The optical axis may be disposed perpendicular to the baseplane 36, or it may be canted as described in U.S. Pat. No. 4,588,258 toHoopman and U.S. Pat. No. 5,138,488 to Szczech. Retroreflection canoccur when light incident on base plane 36 is internally reflected froma first lateral face 31a to a second face 31b, and then to a third face31c, and then back through base 36 toward the light source. In additionto defining a single cube-corner by a three-sided pyramid having atriangular base plane such as disclosed in the Hoopman patent, thecube-corner elements may be defined by a rectangular base, tworectangular sides, and two triangular sides such that each structure hastwo cube-corners each such as disclosed in U.S. Pat. No. 4,938,563 toNelson et al., or may be of essentially any other cube-corner shape andmay be defined by two perpendicular intersecting sets of parallelgrooves (see also U.S. Pat. No. 4,895,428 to Nelson et al.).

FIG. 4 shows the structured surface or backside of a cube-cornersheeting 60, that includes a unitary or single layer of an array ofcube-corner elements 30, like the element depicted in FIG. 3. Eachcube-corner element 30 meets, but is not necessarily connected to, anadjacent cube-corner element at a base edge 35. The array includes threesets of generally parallel grooves 45, 46, and 47. The external dihedralangles (α, FIG. 5) between faces 31 of adjacent cube-corner elements 30vary along the grooves 45-47 in the array. The cube-corner elements inthe array are randomly tilted, and because of this, the apex 34 of onecube, such as cube 30a may be relatively close to another apex such ascube 30b, but cube 30b's apex may then be farther away from anotheradjacent apex such as the apex of cube 30c. Glittering cube-cornersheetings produced from molds of the invention are described in U.S.patent application Ser. No. 08/640,326 entitled "Glittering Cube-ComerRetroreflective Sheeting" filed on the same day as this application(attorney docket number 52373USA3A).

FIG. 5 also illustrates the position of one cube apex relative toanother and additionally shows how the cube's base edges 35 do not liein the same common plane. The base edge 35 of one cube may be disposedcloser to or further away from the front surface 51 of retroreflectivesheeting 60 than the base edges of other adjacent cube-corner elements.And in a single cube, points on one of its base edges 35 may be locatedcloser to or farther away from front surface 51 than points on anotherbase edge 35 in the same cube. Base edges 35 define the lowest point ofgrooves 45-47--and because edges 35 do not all lie in the same plane,the grooves have a varying pitch along their length. If the cube-cornersheeting possesses a land layer 56, it too is also not uniformly spacedfrom the front surface 51. When the cube-corner elements are tilted, thebase planes 36 (FIG. 3) of each cube-corner element are not parallel,and they do not reside in the same plane. Many of the base planes alsodo not reside in the same plane as the front or base surface 51--thatis, the base planes are not parallel to the sheeting's front surface 51when the sheeting is laid flat on a surface.

Cube-coiner element sheetings have been produced where some of theelement's base planes do not reside parallel to the sheeting's frontsurface when the sheeting is laid flat. Such sheetings, however, havehad the array of cube-corner elements disturbed or rearranged in certainareas by sealing a film to the backside of the array (such as discussedbelow with reference to FIGS. 8 and 9) or by creating bubbles (U.S. Pat.No. 5,485,311 to McAllister). The seal line and the bubbles upset thesheeting's front surface and the orientation of the cube-corner elementsin the array. For purposes of this invention, therefore, a sheeting isnot considered to be "laid flat" in those areas where the sheeting isdisturbed by seal lines (item 64 FIGS. 8 and 9) or bubbles (24 of the'311 patent). The base planes 36 (FIG. 3) in sheetings of the inventionmay be offset at angles of zero to 90 degrees from the reference planeor front surface when the sheeting is laid flat. The base planes thatare tilted relative to the front surface of the sheeting when laid flattypically form an angle of about 1 to 10 degrees from the front surface.

FIG. 5 also shows the external dihedral angle, α, that defines the anglebetween faces 31 (FIG. 4) of adjacent cube-corner elements 30. Angle αmay vary along some or all grooves in a single generally parallel grooveset, it may vary along some or all grooves in two generally parallelgroove sets, or it may vary along some or all grooves in all threegenerally parallel groove sets in the array. In an array of randomlytilted cube-corner elements, angle α varies randomly amongst adjacentfaces of adjacent cube-corner elements throughout essentially the wholearray that is intended to glitter. Angle α may vary from zero degrees to180 degrees, but on average ranges from about 35 to 115 degrees fordihedral angles between faces of adjacent cubes.

FIG. 6 illustrates some typical distances of apexes 34 and grooveintersections from the sheeting's front surface 51 (FIG. 5). Thecube-corner element in the upper left hand corner of the array has anapex that is spaced 350 micrometers from the front surface 51. Thefourth cube over from the upper left-hand corner, however, has an apexheight of 335 micrometers. There is thus a difference in apex height of15 micrometers between cubes that are fairly close to one another. Thecube-corner elements typically have an average height of about 20 to 500micrometers, more typically of about 60 to 200 micrometers. Forcube-corner elements that are about 60 to 200 micrometers high, thevariation in height between adjacent apexes typically is about 0 to 60micrometers and typically is about 1 to 40 micrometers on average, moretypically 5 to 25 micrometers on average, but preferably does not exceedmore than 50 micrometers on average. The variation in height betweenadjacent groove intersections for such cubes typically is about 0 to 100micrometers and typically is about 3 to 50 micrometers on average, butpreferably does not exceed more than 60 micrometers on average.

The body layer 58 (FIG. 5) in body portion 54 (FIG. 5) typically has anaverage thickness of approximately 20 to 1200 micrometers, andpreferably is about 50 to 400 micrometers. The optional land layer 56(FIG. 5) preferably is kept to a minimal thickness of 0 to less thanabout 100 micrometers.

In the cube-corner element array shown in FIGS. 4-6, the groove sets 45,46, and 47 are illustrated as being parallel. It is within the scope ofthis invention, however, for grooves of the same set to be other thanparallel. Some grooves may be parallel and others may not. Some groovesmay run parallel to adjacent grooves of the same groove set in someregions of the sheeting but may also cross paths or overlap those samegrooves. In such instances, the cube-corner elements may pile up on eachother. As long as there are two or more grooves that extend in the samegeneral direction roughly parallel to each other, those grooves areviewed as being "generally parallel" regardless of whether the groovesat some other point cross paths, overlap, converge, or diverge.

FIG. 7 shows cube-corner elements intersected by a plane that isparallel to the retroreflective sheeting's front surface 51 (FIG. 5). Asillustrated, the plane intersects the cube-corner elements to producetriangles 62 of different cross-sectional areas. Some cubes may betilted to such an extent that the intersecting plane only passes througha tip of the cube, resulting in a small triangularcross-section--whereas, a cube that stands upright may be intersectedsuch that the triangle resulting from the cross-section is relativelylarge. Thus, even though the cube-corner elements in the array may be ofsimilar size, they can produce triangles of random sizes whenintersected as described because of the manner in which the cubes aretilted with respect to a reference plane.

FIG. 8 shows a retroreflective product 61 that has a seal film 63disposed over the backside of cube-corner elements 30. The seal film isbonded to the body layer 58 of the sheeting 60 through the layer ofcube-corner elements 30 by a plurality of seal lines 64. The bondingpattern produces a plurality of hermetically-sealed chambers 65 thatmaintain a cube/air interface and prevent moisture and dirt fromcontacting the backside of the cube-corner elements. Maintenance of thecube/air interface is necessary to prevent loss of retroreflectivity.

The seal film may be bonded to the retroreflective sheeting using knowntechniques; see for example, U.S. Pat. No. 4,025,159. Sealing techniqueexamples include radio frequency welding, thermal fusion, ultrasonicwelding, and adhesive bonding. When applying a seal film to the backsideof a retroreflective sheeting, considerable attention must be paid tothe composition and physical properties of the seal film. The seal filmmust be able to bond securely to the sheeting, and it should not containcomponents that could adversely affect retroreflectivity or theappearance of the retroreflected product. For example, the seal filmshould not contain components that could leach out (e.g., dyes) tocontact the backside of the cube-corner elements. The seal filmtypically comprises a thermoplastic material because such materials lendthemselves to fusing through relatively simple and commonly availablethermotechniques.

Radio frequency ("RF") welding accomplishes sealing using radiofrequency energy that heats the polymer. When a radio frequency field isapplied to a thermoplastic polymer with polar groups, the tendency ofthe polar groups to switch orientation with the radio frequencydetermines the degree to which RF energy is absorbed and converted tokinetic motion. The kinetic energy is conducted as heat to the entirepolymer molecule, and if enough RF energy is applied, the polymer willheat sufficiently to soften. Detailed discussions of RF welding may befound in U.S. patent application Ser. No. 08/472,444 filed Jun. 7, 1995and in the article, "RF Welding and PVC and Other ThermoplasticCompounds" by J. Leighton, T. Brantley, and E. Szabo in ANTEC 1992, pp.724-728.

A seal film also may be secured to the retroreflective sheeting throughthermal fusion which involves pressing thermoplastic materials togetherbetween heated dies or platen surfaces. The contact forms the desiredsealing pattern. While the heated die or platen surfaces press thethermoplastic materials together, the polymer areas that are in contactmelt and the polymer molecules flow together while hot and form a fusionbond on cooling.

An alternative to radio frequency welding and thermal fusion methods isultrasonic welding. Ultrasonic welding is a technique where twomaterials are bonded together between a horn and an anvil. The hornvibrates at ultrasonic frequencies, commonly in the range of about20,000-40,000 Hz. Pressure is applied to the cube-corner sheeting andthe seal film, and the vibrational energy is dissipated as heat. Thefrictional heating softens the polymer molecules to create a fusion bondbetween the sheeting and the film. The horn and anvil are positioned tolocalize heat in the area where the bond is intended. Heat localizationassures softening and melting of the bonding materials in very smallregions which, in turn, helps minimize damage to the surroundingmaterial from heat exposure.

Amorphous materials that have broad softening ranges may beultrasonically bonded better than crystalline materials because theformer tend to dissipate frictional heat more effectively. Examples ofmaterials that form good-to-excellent ultrasonically welded bondsinclude nylon, polycarbonate, plasticized poly(vinyl chloride) (PVC),polystyrene, thermoplastic polyester, polypropylene, and acrylics.Polyethylene and fluoropolymers are examples of materials that form fairto poor ultrasonic welds.

Ultrasonic welding is sensitive to other factors including plasticvariation from batch to batch, molding parameter changes, moistureabsorption, mold release, lubricants, fillers, regrind, flameretardants, pigments, and plasticizers. Reference is made to thefollowing articles: "Heating and Bonding Mechanisms in UltrasonicWelding of Thermoplastics" by M. N. Tolunay, P. R. Dawson, and K. K.Wang in Polymer Engineering and Science, September 1983, Vol. 23, No.13, p. 726; "Update on Welding: More Science, Less Art" by M. Rogers inPlastics Technology, June 1981, pp. 56-62; "Ultrasonic Welding" inEngineering Materials and Design, April 1981, pp. 31-34.

Adhesive bonding can be achieved by coating an adhesive onto acube-corner sheeting's backside and then bringing the seal film intocontact with the adhesive coated sheeting. Alternatively, the seal filmmay be coated with an adhesive before bonding to the cube-cornersheeting. Adhesive coating may be done in essentially any desiredpattern such that the areas not coated with adhesive formretroreflective cells 65 as shown in FIG. 8. The adhesive also may becoated over a reflective coating that is disposed on the back side ofthe cube-corner sheeting. See U.S. Pat. No. 5,376,431 to Rowland for adescription of adhesive bonding.

When the glittering sheeting is sealed piecewise, the radio frequencytechnique is preferred because the process is generally practiced as a"step and repeat process" that is compatible with sealing individualitems. When the glittering sheeting is sealed continuously from rollgoods, ultrasonic welding is preferred because this process can beeasily practiced as a continuous method.

FIG. 9 illustrates an example of a seal pattern that may be used toproduce a retroreflective product 61. As shown, retroreflective product61 is in the form of a strip that has a length dimension thatsubstantially exceeds the width dimension. Bond lines 64a and 64b aredisposed along the lengthwise edges of sheeting 61 to preventdelamination of seal film 63 (FIG. 8). Disposed laterally inward frombond lines 64a and 64b are bond lines 64c and 64d that run parallel tobond lines 64a and 64b. Extending between bond lines 64c and 64d arebond lines 64e that are not parallel to the sheeting's lengthwise edges.Bond lines 64c-64e define a number of completely enclosed geometricpatterns 67 that define the hermetically sealed chambers 65 shown inFIG. 8. The surface area of the geometric patterns 67 may varydepending, for example, on the width of product 61 but typically areabout 0.5 to 30 cm², more typically about 1 to 20 cm².

Retroreflective product 61 typically comes in sizes ranging fromone-half inch (1.27 cm) to three inches (7.6 cm) wide. Typical widthsare one-half inch (1.27 cm) wide, three-quarters of an inch (1.9 cm)wide, one inch (2.54 cm) wide, one and three eighths inches (3.5 cm)wide, one and one-half inches (3.81 cm) wide, two inches (5.08 cm) wide,or two and three fourths inches (7.0 cm) wide. Lengths of product 61 maytypically be as large as about 100 meters, with the product beingsupplied in roll form.

Panels of retroreflective products that have seal films disposed thereonalso may be produced. Panel sizes may be, for example, 200 cm² to 1000cm². The whole area within the panel, or certain selected areas, withinit, may glitter.

In a typical retroreflective product 61, essentially the whole areawithin the enclosed geometric pattern displays the glittering effect,where each point of light is referenced by number 68. If desired, somegeometric patterns may display the glittering effect while others donot. For example, it may be possible using molds of the invention tohave the triangular patterns 67 alternate between glittering andnon-glittering. It may also be possible to provide glittering portionsor images within each geometric pattern as described below in detail.Although the glittering effect typically would not be noticeable, orsignificantly noticeable, within each seal line because the cube-cornerelements typically become engulfed in the seal line, the glitteringeffect is very noticeable "substantially beyond" the seal line(s). Thatis, the glittering effect may be noticed at a distance beyond where heatand/or pressure from the sealing operation would affect the cube-cornerelements in the array. Typically, a sealing operation that used heatand/or pressure would not affect the cube-corner elements at a distancegreater than two millimeters (mm), and more typically at 5 mm or morefrom a seal line. Sheetings made from a mold of the invention arecapable of glittering across an array of cube-corner elements regardlessof whether a seal film is bonded to the backside of the cube-cornerelement array.

In lieu of (or possibly in addition to) a seal film 63, a reflectivecoating such as a specularly reflective metallic coating can be placedon the backside of the cube-corner elements to promote retroreflection;see, for example, U.S. Pat. Nos. 5,272,562 to Coderre and 5,376,431 toRowland and in WO 93/14422.

The metallic coating may be applied by known techniques such as vapordepositing or chemically depositing a metal such as aluminum, copper,silver, or nickel. Instead of a metallic coating, a layer of dielectricmaterial may be applied to the back side of the cube-corner elements,see, for example, U.S. Pat. Nos. 4,763,985 and 3,700,305 to Bingham.

Although placing a metal coating on the backside of the cube-cornerelements can reduce the sheeting's daytime lightness, the glitteringeffect can counter this reduction. Metal coated glittering samples maydemonstrate LFY lightness values of at least 10, and even greater than25.

FIG. 10 illustrates an example of an article of clothing onto which aretroreflective product 61 may be disposed. The article of clothing maybe a safety vest 69 that has glittering retroreflective products 61secured to its outer surface 70. Other vests that may displayretroreflective products are shown, for example, in U.S. Pat. Nos.5,478,628, Des. 281,028, and Des. 277,808. Examples of other articles ofclothing onto which the retroreflective products of the invention may beapplied include shirts, sweaters, jackets, coats, pants, shoes, socks,gloves, belts, hats, suits, one-piece body garments, bags, backpacks,helmets, etc. The term "article of clothing" thus, as used here, meansany article sized and configured to be worn or carried by a person andis capable of displaying a retroreflective article on its outer surface.

The inventive glittering cube-corner sheetings can be made in accordancewith two techniques. In the first technique, a glittering cube-cornerretroreflective sheeting is made by providing a first cube-cornersheeting that has the cubes arranged in a conventional configuration,namely, a non-random orientation, and exposing this sheeting to heat,pressure, or a combination of both. In the second technique, a mold isproduced that is a negative of a cube-corner sheeting of the invention.This mold may then be used in accordance with the present invention toprovide glittering retroreflective sheetings. A method of makingglittering retroreflective sheetings is described in U.S. patentapplication Ser. No. 08/641,129 entitled "Method of Making GlitteringRetroreflective Sheetings" filed on the same day as this application(attorney docket number 52374USA1A).

When using the first technique, a retroreflective sheeting is firstproduced or otherwise obtained which has the cube-corner elementsarranged in an ordered configuration. There are many patents thatdisclose retroreflective sheetings that have ordered arrays ofcube-corner elements: see, for example, U.S. Pat. Nos. 5,236,751,5,189,553, 5,175,030, 5,138,488, 5,117,304, 4,938,563, 4,775,219,4,668,558, 4,601,861, 4,588,258, 4,576,850, 4,555,161, 4,332,847,4,202,600, 3,992,080, 3,935,359, 3,924,929, 3,811,983, 3,810,804,3,689,346, 3,684,348, and 3,450,459. Ordered cube-corner arrays may beproduced according to a number of known methods, including thosedisclosed in the patents cited in the previous sentence. Other examplesare disclosed in U.S. Pat. Nos.: 5,450,235, 4,601,861, 4,486,363,4,322,847, 4,243,618, 3,811,983, 3,689,346, and in U.S. patentapplication Ser. No. 08/472,444 filed Jun. 7, 1995.

Preferably, the cube-corner elements that are used in the non-randomlyoriented starting sheeting are made from materials that are harder thanthe materials used in the body portion, particularly the body layer. Aselection of such materials allows the cube-corner elements to tilt,without significantly distorting each cube's shape, when the sheeting isexposed to certain amounts of heat and/or pressure. The heat, pressure,or both that are applied to the sheeting should be sufficient to alterthe array significantly from its ordered configuration. With a very softbody layer, pressure alone, that is, pressure above atmospheric, or heatalone, namely, heat greater than the softening temperature may besufficient to change the array from an ordered configuration.

A cube-corner retroreflective sheeting that has hard cubes and a softerbody layer is disclosed in U.S. Pat. No. 5,450,235 to Smith et al. Asdescribed in this patent, the body portion includes a body layer thatcontains a light transmissible polymeric material that has an elasticmodulus less than 7×10⁸ pascals. The cube-corner elements, on the otherhand, contain a light transmissible polymeric material that has anelastic modulus greater than 16×10⁸ pascals. When a cube-corner sheetingmade from materials of those designated elastic modulus values isexposed to certain amounts of heat and pressure, the body layer softens,allowing the cubes to move in response to the pressure and thus becometilted relative to the sheeting's front surface. When using such aconstruction, the land layer (56, FIG. 7) ideally is kept to a minimalthickness (for example, less than ten percent of the cube-corner elementheight), and preferably zero thickness, so that the cubes can easilytilt along their base edges. For this same reason, it is also preferredin this invention that the cube-corner elements are fractured alongtheir base edges as disclosed in U.S. patent application Ser. No.08/139,914 filed Oct. 20, 1993 and in U.S. patent application Ser. No.08/472,444 filed Jun. 7, 1995. U.S. patent application Ser. No.08/472,444 also discloses a number of materials that may be used toproduce cube-corner sheetings in accordance with this invention. Thispatent application specifies that the elastic modulus of the cube-cornerelements is at least 1×10⁷ Pascals greater than the elastic modulus ofthe body layer and that its cube-corner elements may be made frommaterials that have an elastic modulus greater than about 2.0×10⁸pascals (preferably greater than about 25×10⁸ pascals) and that the bodylayer or overlay may be made from materials that preferably have anelastic modulus less than about 13×10⁸ pascals.

Elastic modulus may be determined according to standardized test ASTM D882-75b using Static Weighing Method A with a five inch initial gripseparation, a one inch sample width, and an inch per minute rate of gripseparator. Under some circumstances, the polymer may be so hard andbrittle that it is difficult to use this test to ascertain the modulusvalue precisely (although it would be readily known that it is greaterthan a certain value). If the ASTM method is not entirely suitable,another test, known as the "Nanoindentation Technique" may be employed.This test may be carried out using a microindentation device such as aUMIS 2000 available from CSIRO Division of Applied Physics Institute ofIndustrial Technologies of Lindfield, New South Wales, Australia. Usingthis kind of device, penetration depth of a Berkovich pyramidal diamondindenter having a 65 degree included cone angle is measured as afunction of the applied force up to the maximum load. After the maximumload has been applied, the material is allowed to relax in an elasticmanner against the indenter. It is usually assumed that the gradient ofthe upper portion of the unloading data is found to be linearlyproportional to force. Sneddon's analysis provides a relationshipbetween the indenting force and plastic and elastic components of thepenetration depth (Sneddon I. N. Int. J Eng. Sci. 3, pp. 47-57 (1965)).From an examination of Sneddon's equation, the elastic modulus may berecovered in the form E/(1-v²). The calculation uses the equation:

    E/(1-v.sup.2)=(dF/dh.sub.e)F.sub.max 1/(3.3h.sub.pmax tan (θ))

where:

v is Poisson's ratio of the sample being tested;

(dF/dh_(e)) is the gradient of the upper part of the unloading curve;

F_(max) is the maximum applied force;

h_(pmax) is the maximum plastic penetration depth;

θ is the half-included cone angle of the Berkovich pyramidal indenter;and

E is the elastic modulus.

It may be necessary to correlate the results of the nanoindentationtechnique back to the ASTM method.

FIG. 11 illustrates how to prepare a glittering cube-corner sheetingusing heat and/or pressure in a batchwise process. Using this technique,a cube-corner retroreflective sheeting that contains an ordered array ofcube-corner elements such as sheeting 10 may be placed in a platen pressor laminator 71 that includes first and second pressure-applyingsurfaces 72 and 74. The laminator may be, for example, a Hix model N-800heat transfer machine available from Hix Corporation of Pittsburg, Kans.

A Hix N-800 laminator has a first pressure-applying surface 72 that ismade of metal and that may be heated to temperatures as high as 500° F.The second pressure-applying surface 74 is an unheated rubber mat. Inoperation, two layers of release paper 76 may optionally be disposedbetween the surfaces 72 and 74 and the cube-corner sheeting 10. Acarrier 78 (such as made from polyester) may be disposed on thecube-corner sheeting's front surface 51. Carrier 78 is a byproduct ofthe process used to produce sheeting 10 (see, for example, U.S. patentapplication Ser. No. 08/472,444 at the discussion describing its FIG. 4,where the carrier is represented by numeral 28) and may optionallyremain thereon until after the cube-corner elements have been rearrangedfrom exposure to heat and/or pressure.

When the ordered non-glittering cube-corner sheeting and optionalrelease paper 76 are arranged in the heat lamination machine as shown inFIG. 11, the machine is activated so that the pressure-applying surfaces72 and 74 move toward each other and hold the ordered cube-cornersheeting at a desired temperature and pressure for a predetermined time.If desired, the lower release paper 76 in FIG. 11 may be omitted, andthe pattern or image of the lower, unheated surface 74 of the heatlaminating machine may be transferred to the retroreflective sheeting ina glittering pattern. In lieu of a laminating machine, a vacuumformer--such as a Scotchlite™ Heat Lamp Applicator available from DaycoIndustries, Inc., Miles, Mich.; P. M. Black Co., Stillwater, Minn.; andConverting Technologies, Inc., Goodard, Kans.--may be used.

The amount of heat and/or pressure applied to a cube-corner sheeting 10may vary depending on the materials from which the cube-corner sheetingis made. When polymeric materials having an elastic modulus of about10×10⁸ to 25×10⁸ are used in the cube-corner elements 12 (and anoptional land layer 16), and a polymeric material having an elasticmodulus of about 0.05×10⁸ to 13×10⁸ pascals is used in the body layer18, the cube-corner sheeting, preferably, is heated to a temperature ofabout 300° to 400° F. (150° to 205° C.) and that about 7×10⁴ to 4.5×10⁵pascals (10 to 60 psi) of pressure are applied to the article. Moreparticularly, when cube-corner elements are employed that are made from1,6-hexanediol diacrylate, trimethylolpropane triacrylate, bisphenol Aepoxy diacrylate in a ratio of 25 parts to 50 parts to 25 parts,respectively, and containing one weight percent (based on resin weight)of Darocur.sup.™ 4265 photoinitiator (Ciba Geigy) and having an elasticmodulus of about 16×10⁸ to 20×10⁸ pascals, and the body layer is madefrom a plasticized poly(vinyl chloride) film having an elastic modulusof around 0.2×10⁸ to 1×10⁸ pascals, the cube-corner sheeting preferablyis exposed to temperatures of about 320° to 348° F. (160° to 175° C.)and pressures of about 1.4×10⁵ to 2.8×10⁵ pascals (20 to 40 psi). Usingpolymers that have a relatively high elastic modulus, for example,greater than 16×10⁸ pascals, the geometry of each cube, namely, itsinternal dihedral angles, are generally maintained to within a couple ofdegrees.

In FIG. 12, a continuous method is shown for applying heat and/orpressure to a retroreflective sheeting 10 to produce a glitteringsheeting 60. In this method, the retroreflective sheeting 10, having theoptional carrier film 78 disposed thereon, is fed through the nip formedby rolls 77 and 77'. As shown, cube-corner elements 12 are in anon-random, ordered configuration before being exposed to the heatand/or pressure from rolls 77 and 77', but after exiting the rolls theyare randomly tilted, and the dihedral angles formed between adjacentcube-corner elements vary along each groove in the array. The baseplanes of each cube-corner element also do not reside in the samegeneral plane. The sheeting 60 that exits the rolls is capable ofproducing a glittering effect, whereas the cube-corner sheeting 10 thathas not been exposed to sufficient amounts of heat and/or pressure isincapable of producing such an effect. The amounts of heat and/orpressure that may be used in this continuous method are similar to thoseused in the batchwise method for similar starting materials. When usingheat, either or both rolls 77 and 77' may be heated to the temperaturesufficient to alter the cube configuration.

In the second technique for producing a glittering cube-cornerretroreflective sheeting, a mold may be used that is a negative of aglittering cube-corner sheeting. Such a mold may be made from aglittering cube-corner retroreflective sheeting that is produced by thefirst technique described above. That is, the structured surface orbackside of an array of, for example, randomly-tilted cube-cornerelements can be used as a pattern to produce the mold. This can beaccomplished, for example, by depositing suitable mold material(s) ontothe structured side of an array of randomly tilted cube-corner elementsand allowing the mold material(s) to harden in place. The randomlytilted cube-corner sheeting that is used as the pattern may then beseparated from the newly formed mold. The mold is then capable ofproducing cube-corner sheetings that glitter.

Molds or tools are commonly made from hard materials such as metals.These opaque molds may be retroreflective and often may be glitteringthemselves. Molds may also be made from polymeric materials. These moldsmay be transparent, translucent, or opaque and may exhibit theglittering effect. For a mold that possesses cube-corner elements thatare oriented to produce sheetings exhibiting intense glittering, thearray of cube-corner elements in the mold may be arranged in such ahighly random and complex configuration, for example, cube-cornerelements piled up upon each other in the array--that it is difficult toseparate the sheeting product from the mold. Molds made from softmaterials such as polysiloxanes and vinyl type plastisols may be usefulfor assisting the release of glittering sheeting comprising hard orotherwise difficult resin systems. Examples of molds prepared from suchsoft materials include those made from Sylgard™ 184 polysiloxaneelastomer (Dow Corning, Midland, Mich.) and from vinyl type plastisols,for example, mixtures of two parts Shore A 90 durometer clear vinylplastisol and one part M2112 clear vinyl plastisol (Shore A 10durometer, available from Plast-O-Meric, Sussex, Wis. A preferred softmold is made from a polymeric material(s) having a durometer of lessthan about Shore A 90 and if desired less than about 60 for specialcases.

As an alternate method of producing a mold of the invention, a diamondtool may be used to fashion the array of cube-corner elements. This maybe accomplished by, for example, using a number of diamond cuttingtools, each tool being able to cut the groove which forms one of thedesired dihedral angles between adjacent cube-corner elements. Groovedepth and angle between adjacent cube-corner element faces in any singlegroove is determined by the profile of the diamond cutting tool that isused to cut the mold material.

To prepare a mold having cube-corner elements with varying dihedralangles between faces of adjacent cube-corner elements along the groove,it is necessary to position a diamond cutting tool capable of cuttingthe first desired dihedral angle, insert it into the mold material andcut the groove portion that extends from one groove intersection to theadjacent groove intersection. The tool is then removed from the moldmaterial, and the diamond cutting tool is replaced by a tool that iscapable of cutting the next desired dihedral angle along the groove. Thenewly selected tool is then positioned in the growing groove as close aspossible to the location where the first cutting tool finished cutting.Cutting the groove is then continued with the second cutting tool untilthe next groove intersection is reached. The second cutting tool is thenremoved from the mold material and replaced with a cutting tool capableof cutting the third desired dihedral angle in preparation for cuttingthe next groove portion. This process is continued for the length of thegroove. After completion of the first groove, the next or adjacentgroove may be cut in the same manner using various cutting tools andincremental cuts until the desired number of parallel, or generallyparallel, grooves have been completed.

After the first set of grooves is complete, the diamond cutting tool isadjusted so that the second set of parallel grooves may be cut such thatthey intersect with the first set and contain varying dihedral anglesbetween adjacent cube-corner faces. This process is continued until thedesired number of sets of generally parallel grooves are cut into themold material.

A mold of the invention also may be produced using pin techniques. Moldsmanufactured using pin bundling are made by assembling togetherindividual pins that each have an end portion shaped with features of acube-corner retroreflective element. U.S. Pat. No. 3,632,695 to Howelland U.S. Pat. No. 3,926,402 to Heenan et al. disclose illustrativeexamples of pin bundling. A plurality of pins are typically fashioned tohave an optically active surface on one end disposed at an oblique angleto the longitudinal axis of the pin. The pins are bundled together toform a mold having a structured surface in which the optical surfacescombine to form the cube-corner elements. The mold may be used to formretroreflective sheeting or to generate other molds useful inmanufacturing cube-corner sheeting. Pins may be arranged such that thedihedral angle between optical faces of adjacent cube-corner elementsvary. One advantage associated with pin bundling techniques is that thedihedral angle may be varied in a single groove set, or in two or moregroove sets. The pins also can be configured such that there are nogenerally parallel grooves and/or such that the cube-corner elements donot possess base planes that are parallel to one another when theresulting sheeting is laid flat. Pin bundling thus can provideadditional flexibility in producing glittering retroreflective sheeting.

FIG. 13 illustrates a mold 79 according to the invention that is anegative of an array of cube-corner elements that comprise a glitteringretroreflective sheeting. The mold (also referred to in the art as atool) therefore may possess three sets of generally parallel v-shapedgrooves 85, 86, and 87, and the planar faces 81 of adjacent cube-cornerelements 80 can form dihedral angles that vary in dimension along eachgroove in the mold's array. For example, in groove 86a, faces 81a and81b of adjacent cubes 80a and 80b form a tighter dihedral angle α (FIG.5) than faces 81c and 81d of cubes 80c and 80d. The mold may beessentially the same as the array of cube-corner elements of theinvention with the exception of being a negative thereof, and since itmay not need to transmit light or be conformable, it may be made from anopaque material that is relatively inflexible, for example, metal.

FIG. 14 schematically shows how a structured article that is capable ofglittering and retroreflecting light may be formed from a mold 79 of theinvention. The method includes an apparatus, shown generally as 90, forcasting and curing composite sheeting 60. As shown, body layer 58 isdrawn from a roll 92 to a nip roller 93 such as a rubber coated roller.At roller 93, the body layer 58 contacts a suitable resin formulation 94previously applied to an endless patterned mold 79 on a roll 95 (orother suitable endless carrier that forms a loop, e.g. a belt) through acoating die 96. The excess resin 94 extending above the cube-cornerelements 80 may be minimized by setting nip roller 93 to a width settingthat is effectively less than the height of the cube-corner formingelements of mold 79. In this fashion, mechanical forces at the interfacebetween nip roller 93 and mold 79 ensure that a minimum amount of resin94 extends above the mold elements 80. Depending on its flexibility, thebody layer 58 may be optionally supported with a suitable carrier film78 that provides structural and mechanical integrity to the body layer58 during casting and curing, and which is stripped from the body layer58 after the sheeting is removed from the mold 79 at roll 98. Use of acarrier film 78 is preferred for low modulus body layers 58.

The method shown in FIG. 14 may be altered such that the resin 94 isapplied to the body layer 58 first rather than being first deposited onthe mold 79. This embodiment for a continuous process is discussed inU.S. patent application Ser. No. 08/472,444 with reference to its FIG.5.

As shown in FIG. 14, the resin composition that forms the array ofcube-corner elements can be cured in one or more steps. Radiationsources 99 expose the resin to actinic radiation, such as ultravioletlight or visible light, depending upon the nature of the resin, in aprimary curing step. The actinic radiation from source 99 irradiates theresin through the layer 58--thus imposing a requirement that the bodylayer 58 transmit radiation to allow curing to occur. Alternatively,curing can be performed by irradiation through the mold 79--if the moldused is sufficiently transparent to transmit the selected radiation.Curing through both the tool and the body layer also may be carried out.

The primary curing may completely cure the cube-corner elements, or maypartially cure the resin composition to a degree sufficient to producedimensionally stable cube-corner elements that no longer require thesupport of the mold 79. The sheeting 60 can then be removed from themold 79, exposing the sheeting's cube-corner elements 30. One or moresecondary curing treatments 100, selected depending upon the nature ofthe resin, can then be applied to fully cure the array of cube-cornerelements and strengthen the bond between the array of cube-cornerelements and the body layer. This bifurcated curing approach can permitoptimized processing and materials selection. For instance, a sheetingmade from a body layer that contains an ultraviolet absorber (to impartgreater durability and weathering ability) can be made by applying aprimary curing treatment of visible light through thelight-transmissible body layer, and then removing the sheeting from themold 79 at roll 98 and applying a second curing treatment 100 ofultraviolet radiation to the exposed cube-corner elements. Such abifurcated approach may permit faster overall production.

The extent of the second curing step depends on a number of variables,among them the rate of feed-through of the materials, the composition ofthe resin, the nature of any crosslinking initiators used in the resinformulation, and the geometry of the mold. In general, faster feed ratesincrease the likelihood that more than one curing step is needed.Selection of curing treatments depends in large part on the specificresin chosen for producing the cube-corner elements. Electron beamcuring could be used, for example, in lieu actinic radiation.

Thermal curing materials also may be used when making glitteringretroreflective sheeting from a mold of the invention. In this case, themold is heated to a temperature sufficient to cause development ofenough cohesion in the newly formed glittering cube-corner material toallow it to be removed from the mold without damaging the physical oroptical properties of the newly formed sheeting. The selectedtemperature is a function of the thermal curing resin. Thermal curingmay be achieved, for example, by heating the resin, by heating the mold,or by heating the glittering sheeting by indirect means.

Combinations of these methods also may be used. Indirect heatingincludes methods such as heating with lamps, infrared or other heatsource filaments, or any other convenient method. The mold may also behoused in an oven or other environment that is maintained at thetemperature required by the thermal curing resin selected.

After the glittering retroreflective sheeting has been removed from themold, it may be further treated by exposure to heat from an oven orother heated environment. Such subsequent heat treatment may adjust thesheeting's physical or other properties to some desired state, completereactive processes in the sheeting, or remove volatile substances suchas solvents, unreacted materials, or by-products of the thermal curingsystem.

Thermal curing resins may be applied to the mold as solutions or as neatresin formulations. Resins also may be either reactively extruded orextruded in the molten state onto the mold. Methods of thermal curingafter applying the resins to the mold, and any subsequent exposure ofthe sheeting to heat, may be done independent of applying the thermalcuring resin to the mold.

An advantage of glittering retroreflective sheeting made from thermalcuring materials in a mold is that both the cube-corner elements 30(FIG. 3) and body portion 54 (FIG. 5) may be made from the samesubstance, which may be applied to the mold in a single operation. Aconsequence of this construction is that the sheeting may exhibituniform materials and properties throughout the sheeting. A furtheradvantage is that constructions of this type do not require a separatebody layer to be applied as illustrated in FIG. 14.

In addition to curing treatments, sheeting may also be heat treatedafter it is removed from the mold. Heating serves to relax stresses thatmay have developed in the body layer or in the cube-corner elements, andto drive off unreacted moieties and byproducts. Typically, the sheetingis heated to an elevated temperature, for example, above the polymer'sglass transition temperature(s). The sheeting may exhibit an increase inretroreflective brightness after a heat treatment.

In lieu of the above methods, glittering retroreflective sheetings alsomay be produced by embossing a polymeric sheet over a mold thatpossesses cube-corner elements arranged in accordance with the presentinvention. Examples of embossing methods are disclosed in U.S. Pat.Nos.: 5,272,562, 5,213,872, and 4,601,861.

Glittering retroreflective sheetings that display images also may beproduced in accordance with the present invention.

FIG. 15 illustrates a retroreflective article 101 that displays theimage "ABC". The image 102 in this case is characterized by aretroreflective glittering area, while the background 103 ischaracterized by a retroreflective non-glittering area. As used herein,an "image" may be any combination of alphanumeric characters or otherindicia that stands out in contrast to the background. Glittering imagedretroreflective articles, like article 101, may be produced as describedbelow.

Imaged glittering sheeting may be produced in a first embodiment byinserting a material in the shape of the desired image into the assemblyshown in FIG. 11. Thin material in the shape of the desired image, suchas an insert 104 (104 refers generically to any suitable insertincluding 104a and 104b of FIGS. 16a and 16b) in FIG. 11 can be placedbetween the cube-corner reflective elements 30 and the optional lowerrelease liner 76. The image materials may be a polymeric film made from,for example, polyester. The insert 104 may comprise a large, smoothsheet from which the desired image has been cut, forming a negativeimage in the insert. Subjecting this arrangement to processingconditions of elevated temperature and/or pressure results in aretroreflective sheeting that bears the desired image as a glitteringportion on a background that is substantially not glittering or that hasa low level of glittering. When the insert 104 is in the size and shapeof the desired image, subjecting the sheeting 10 to elevated temperatureand/or pressure results in retroreflective sheet material that bears anon-glittering image corresponding to the insert 104 on the glitteringbackground. A preferred embodiment is without the release liner 76.

An insert 104 can be placed with the image forming elements in contactwith exposed cube-corner elements 30 as shown in FIG. 11, or on the topface of the ordered retroreflective sheeting 10 with image formingelements 106 contacting the optional polyester film liner 78 or directlycontacting the front surface 51. Alternatively, an ordered cube-cornersheeting 10 may be inserted in laminator 71 with the cube-cornerelements 30 facing the heated laminator surface 72, and the frontsurface 51 (and optional carrier 78) facing an unheated laminatorsurface 74. Thus, an image forming insert may be disposed either aboveor below the sheeting.

In FIG. 16a an image insert 104a is shown that may comprise a durablematerial 105 that bears projections 106 rising away from the surface ofthe sheet material 105. In this embodiment, the projections 106 form thedesired image. An example of such a device is a flexographic printingplate. When this type of image bearing device is placed in thearrangement of FIG. 11 such that the image forming projections 106 ofinsert 104a contact the exposed cube-corners and the assembly issubjected to elevated temperature and/or pressure, a retroreflectivesheeting is produced that bears a glittering image on a substantiallynon-glittering background.

The degree and extent of glittering may be controlled by the processconditions. For example, processing with a flexographic printing platefor short time periods results in an image that is capable of glitteringonly at the points where projections 106 directly contact the backsideof the cube-corner elements 30. Non-contact areas remain retroreflectiveand substantially not glittering. As processing time increases, and asprocessing temperature increases, the extent of glittering extends awayfrom the contact points of projections 106, and the resulting imagegradually changes from (a) glittering only at contact points to (b) aglittering image on a glittering background, to (c) a non-glitteringimage (where cube-corners have been substantially pushed out of thecontact areas) on glittering background.

In FIG. 16b, an image forming element 104b is shown that may comprise acarrier material 108 on which a heat-transferrable material 110 has beendeposited in the shape and size of the desired image. For example,heat-transferrable ink 110 may be deposited on a carrier film 108 in theform of the image to be transferred. The carrier film 108 bearing thedesired image is placed as an insert 104 in a laminator 71 of FIG. 11such that the exposed back side of the cube-corner elements 30 contactsthe image surface 110 on carrier film 108. The arrangement is subjectedto the processing conditions of elevated temperature and/or pressure,and the resulting retroreflective sheeting bears a non-retroreflectiveimage on a glittering retroreflective background.

The image bearing insert 104 in FIG. 11 also may be a large piece offabric (not shown) or other material bearing an overall pattern ortexture. In the case of a fabric insert, the image carried by the insertis derived from the fabric's configuration. Additionally, the image onthe sheeting may correspond to an image cut from the fabric. When afabric type insert is placed in contact with the exposed back side ofthe cube-corner elements 30 and the arrangement is subjected to elevatedtemperature and/or pressure, the resulting retroreflective cube-cornersheeting bears an overall image that is capable of glittering and thatexhibits the configuration or texture of the fabric. Further, thefabric's texture or weave can enhance the glittering effect in theimaged area. Coarse fabrics tend to encourage more glittering. Ifdesired, the lower release paper 76 in FIG. 11 may be removedcompletely, and the pattern or image of the lower, unheated surface 74of the heat laminating machine may be transferred to the retroreflectivesheeting in a glittering pattern.

There is broad latitude in producing images by contacting the orderedcube-corner retroreflective sheeting with an image forming element. Theappearance of the image depends on process conditions, the constructionfrom which the imaged glittering sheeting is made, and on the size,shape, and materials of the image forming elements. The degree ofglittering in imaged and in nonimaged areas may be successfully alteredwhen one or more of these variables is changed. When the image formingelement 104 is, for example, a textured surface such as fabric--such asa woven polyester mesh--the glittering effect may be considerablyenhanced when compared with the glittering sheeting prepared in theabsence of such a textured surface.

Photomicrographs of sheeting with enhanced glittering showed asubstantially greater degree of cube-corner element reorientation,including groups of cube-corner elements piled upon each other, thansheeting formed in the absence of a textured image forming element. Itis believed that the enhanced glittering effect is related to theadditional reflective paths available to light incident on the piledcube-corner elements. Accordingly, there is a general range ofglittering image forming abilities which can be achieved by changingthese or other variables.

A retroreflective sheeting capable of displaying the glittering effect,prepared from either the first or second technique described above, mayalso be made to bear an image by printing directly onto the outersurface 51 of the body layer 58. When transparent inks are used, theglittering effect and retroreflection are visible through thetransparent image and are dominated by that color. When opaque inks areused, the retroreflection and the glittering effects are blocked only atthe image area when viewed from the front side of the sheeting.Transparent and opaque inks also may be placed on the backside of thecube-corner elements to produce images.

Retroreflective sheetings capable of glittering and bearing images alsomay be prepared by the second technique, directly from a mold.Essentially any method used to prepare retroreflective sheetings thatdisplay glittering images on a non-glittering or glittering backgroundor non-glittering images on a glittering background according to thefirst technique (FIG. 11) is also applicable to the second technique(FIG. 14). When a glittering image is located on a glitteringbackground, the imaged area and the background exhibit varying degreesof glitter so that the imaged area is discernible from the background. Aglittering retroreflective sheeting that displays an image may be usedas a pattern on which mold materials are deposited and/or cured. Removalof the patterned sheeting reveals a newly formed mold that bears theimage formed on the pattern material. Use of such molds producessheeting that is capable of retroreflecting light and that displays theglittering effect and still contains the image applied to the originalsheeting from which the mold was prepared. Images printed, deposited, orformed directly on the exposed back side of the cube-corner elements byvarious techniques may be faithfully replicated in the mold makingprocess. Images placed on the body layer 58 may also end up beingreplicated in the mold making process.

Light transmissible polymeric materials may be used to produce aretroreflective sheetings of the invention. Preferably the selectedpolymers can transmit at least 70 percent of the intensity of the lightincident upon it at a given wavelength. More preferably, the polymerstransmit greater than 80 percent, and still more preferably greater than90 percent, of the incident light.

For some applications, particularly when producing a glittering articleaccording to the first technique (that is, using heat and/or pressure),the polymeric materials that are employed in the cube-corner elementspreferably are hard and rigid. The polymeric materials may be, forexample, thermoplastic or crosslinkable resins. The elastic modulus ofsuch polymers preferably is greater than about 10×10⁸ pascals, and morepreferably is greater than about 13×10⁸ pascals.

Examples of thermoplastic polymers that may be used in the cube-cornerelements include acrylic polymers such as poly(methyl methacrylate);polycarbonates; cellulosics such as cellulose acetate, cellulose(acetate-co-butyrate), cellulose nitrate; epoxies; polyurethanes;polyesters such as poly(butylene terephthalate), poly(ethyleneterephthalate); fluoropolymers such as poly(chlorofluoroethylene),poly(vinylidene fluororide); polyvinyl halides such as poly(vinylchloride) or poly(vinylidene chloride); polyamides such aspoly(caprolactam), poly(amino caproic acid), poly(hexamethylenediamine-co-adipic acid), poly(amide-co-imide), and poly(ester-co-imide);polyetherketones; poly(etherimide); polyolefins such aspoly(methylpentene); poly(phenylene ether); poly(phenylene sulfide);poly(styrene) and poly(styrene) copolymers such aspoly(styrene-co-acrylonitrile),poly(styrene-co-acrylonitrile-co-butadiene); polysulfone; siliconemodified polymers (i.e., polymers that contain a small weight percent(less than 10 weight percent) of silicone) such as silicone polyamideand silicone polycarbonate; fluorine modified polymers such asperfluoropoly(ethyleneterephthalate); and mixtures of the above polymerssuch as a poly(ester) and poly(carbonate) blend, and a fluoropolymer andacrylic polymer blend.

The cube-corner elements also may be made from reactive resin systemsthat are capable of being crosslinked by a free radical polymerizationmechanism by exposure to actinic radiation. Additionally, thesematerials may be polymerized by thermal means using a thermal initiatorsuch as benzoyl peroxide. Radiation-initiated cationically polymerizableresins also may be used.

Reactive resins suitable for forming the cube-corner elements may beblends of a photoinitiator and at least one compound bearing an acrylategroup. Preferably the resin blend contains a difunctional orpolyfunctional compound to ensure formation of a crosslinked polymericnetwork when irradiated.

Examples of resins that are capable of being polymerized by a freeradical mechanism include: acrylic-based resins derived from epoxies,polyesters, polyethers, and urethanes; ethylenically unsaturatedcompounds; aminoplast derivatives having at least one pendant acrylategroup; isocyanate derivatives having at least one pendant acrylategroup; epoxy resins other than acrylated epoxies; and mixtures andcombinations thereof. The term acrylate is used here to encompass bothacrylates and methacrylates. U.S. Pat. No. 4,576,850 to Martensdiscloses examples of crosslinked resins that may be used in thecube-corner elements of glittering retroreflective sheeting.

Ethylenically unsaturated resins include both monomeric and polymericcompounds that contain atoms of carbon, hydrogen and oxygen, andoptionally nitrogen, sulfur and the halogens. Oxygen or nitrogen atomsor both are generally present in ether, ester, urethane, amide and ureagroups. Ethylenically unsaturated compounds preferably have a molecularweight of less than about 4,000 and preferably are esters made from thereaction of compounds containing aliphatic monohydroxy groups oraliphatic polyhydroxy groups and unsaturated carboxylic acids, such asacrylic acid, methacrylic acid, itaconic acid, crotonic acid,isocrotonic acid, maleic acid, and the like.

Some examples of compounds having an acrylic or methacrylic group arelisted below. The listed compounds are illustrative and not limiting.

(1) Monofunctional compounds:

ethylacrylate, n-butylacrylate, isobutylacrylate, 2-ethylhexylacrylate,n-hexylacrylate, n-octylacrylate, isooctylacrylate, isobornyl acrylate,tetrahydrofurfuryl acrylate, 2-phenoxyethyl acrylate,N,N-dimethylacrylamide;

(2) Difunctional compounds:

1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, neopentylglycoldiacrylate, ethylene glycol diacrylate, triethyleneglycol diacrylate,and tetraethylene glycol diacrylate, and diethylene glycol diacrylate;

(3) Polyfunctional compounds:

trimethylolpropane triacrylate, glyceroltriacrylate, pentaerythritoltriacrylate, pentaerythritol tetraacrylate, andtris(2-acryloyloxyethyl)isocyanurate.

Some representative examples of other ethylenically unsaturatedcompounds and resins include styrene, divinylbenzene, vinyl toluene,N-vinyl pyrrolidone, N-vinyl caprolactam, monoallyl, polyallyl, andpolymethallyl esters such as diallyl phthalate and diallyl adipate, andamides of carboxylic acids such as and N,N-diallyladipamide.

Examples of photopolymerization initiators that may be blended with theacrylic compounds include the following illustrative initiators: benzyl,methyl o-benzoate, benzoin, benzoin ethyl ether, benzoin isopropylether, benzoin isobutyl ether, etc., benzophenone/tertiary amine,acetophenones such as 2,2-diethoxyacetophenone, benzyl methyl ketal,1-hydroxycyclohexyl phenyl ketone,2-hydroxy-2-methyl-1-phenylpropan-1-one,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one,2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1-butanone,(2,4,6-trimethylbenzoyl)diphenylphosphine oxide,2-methyl-1-4-(methylthio)phenyl-2-morpholino-1-propanone,bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, etcetera. These compounds may be used individually or in combination.

Cationically polymerizable materials include but are not limited tomaterials containing epoxy and vinyl ether functional groups. Thesesystems are photoinitiated by onium salt initiators such astriarylsulfonium, and diaryliodonium salts.

Preferred polymers for use in the cube-corner elements includepoly(carbonate), poly(methylmethacrylate), poly(ethylene terephthalate),aliphatic polyurethanes and crosslinked acrylates such asmulti-functional acrylates or acrylated epoxies, acrylated polyesters,and acrylated urethanes blended with mono- and multi-functionalmonomers. These polymers are preferred for one or more of the followingreasons: thermal stability, environmental stability, clarity, releasefrom the tooling or mold, or high receptivity for receiving a reflectivecoating.

The polymeric materials employed in a land layer, if one is present, maybe the same as the polymers that are employed in the cube-cornerelements.

The optional land layer preferably is has a smooth interface with thecubes and the body layer. Cavities and/or interfacial roughnesspreferably are avoided between the cubes and the optional land layer orthe body layer so that optimum brightness can be displayed by theretroreflective sheeting when light is retroreflected therefrom. A goodinterface prevents spreading of retroreflective light from refraction.When present, the land layer, in most instances, is integral with thecube-corner elements. By "integral" is meant the land and cubes areformed from a single polymeric material--not two different polymericlayers subsequently united together. The polymers that are employed inthe cube-corner elements and land layer can have refractive indices thatare different from the body layer. Although the land layer desirably ismade of a polymer similar to that of the cubes, the land layer also maybe made from a softer polymer such as those described above for use inthe body layer.

The body layer may comprise a low elastic modulus polymer for easybending, curling, flexing, conforming, or stretching, and for allowingthe cube-corner elements to become reoriented when an ordered array isexposed to heat and pressure. The elastic modulus may be less than 5×10⁸pascals, and may also be less than 3×10⁸ pascals. A low elastic modulusbody layer, however, is not always required. If it is desired to makeglittering retroreflective sheetings which are less flexible, sheetingswith body layer having higher elastic, modulus may be used, such asrigid vinyl with elastic modulus about 21 to 34×10⁸ Pa. Generally, thepolymers of the body layer have a glass transition temperature that isless than 50° C. The polymer preferably is such that the polymericmaterial retains its physical integrity under the conditions to which itis exposed during processing. The polymer desirably has a Vicatsoftening temperature that is greater than 50° C. The linear moldshrinkage of the polymer desirably is less than 1 percent, althoughcertain combinations of polymeric materials for the cube-corner elementsand the body layer may tolerate a greater degree of shrinking of thebody layer polymer. Preferred polymeric materials that are used in thebody layer are resistant to degradation by UV light radiation so thatthe retroreflective sheeting can be used for long-term outdoorapplications. As indicated above, the materials or polymer body layer islight transmissible and preferably is substantially transparent. Bodylayer films with a matte finish--that became transparent when the resincomposition is applied thereto, or that become transparent during thefabrication process, for example, in response to the curing conditionsused to form the array of cube-corner elements--are useful. The bodylayer may be either a single layer or a multi-layer component asdesired. Examples of polymers that may be employed in the body layerinclude:

fluorinated polymers such as: poly(chlorotrifluoroethylene), for exampleKel-F800™ available from 3M, St. Paul, Minn.;poly(tetrafluoroethylene-co-hexafluoropropylene), for example Exac FEP™available from Norton Performance, Brampton, Mass.;poly(tetrafluoroethylene-co-perfluoro(alkyl)vinylether), for example,Exac PEA™ also available from Norton Performance; and poly(vinylidenefluoride-co-hexafluoropropylene), for example, Kynar Flex-2800™available from Pennwalt Corporation, Philadelphia, Pa.;

ionomeric ethylene copolymers such as: poly(ethylene-co-methacrylicacid) with sodium or zinc ions such as Surlyn-8920™ and Surlyn-9910™available from E. I. duPont Nemours, Wilmington, Del.;

low density polyethylenes such as: low density polyethylene; linear lowdensity polyethylene; and very low density polyethylene;

plasticized vinyl halide polymers such as plasticized poly(vinylchloride);

non- or unplasticized rigid vinyl polymers such as Pentaprint™ PR 180from Klockner Pentaplast of America, Inc., Gordonsville, Va.;

polyethylene copolymers including: acid functional polymers such aspoly(ethylene-co-acrylic acid) and poly(ethylene-co-methacrylic acid)poly(ethylene-co-maleic acid), and poly(ethylene-co-fumaric acid);acrylic functional polymers such as poly(ethylene-co-alkylacrylates)where the alkyl group is methyl, ethyl, propyl, butyl et cetera, or CH₃(CH₂)n- where n is 0-12, and poly(ethylene-co-vinylacetate); and

aliphatic and aromatic polyurethanes derived from the following monomers(1)-(3): (1) diisocyanates such asdicyclohexylmethane-4,4'-diisocyanate, isophorone diisocyanate,1,6-hexamethylene diisocyanate, cyclohexyl diisocyanate, diphenylmethanediisocyanate, and combinations of these diisocyanates, (2) polydiolssuch as polypentyleneadipate glycol, polytetramethylene ether glycol,polyethylene glycol, polycaprolactone diol, poly-1,2-butylene oxideglycol, and combinations of these polydiols, and (3) chain extenderssuch as butanediol or hexanediol. Commercially available urethanepolymers include: PN-04, or 3429 from Morton International Inc.,Seabrook, N.H., or X-4107 from B. F. Goodrich Company, Cleveland, Ohio.

Combinations of the above polymers also may be employed in the bodylayer of the body portion. Preferred polymers for the body layerinclude: the ethylene copolymers that contain units that containcarboxyl groups or esters of carboxylic acids such aspoly(ethylene-co-acrylic acid), poly(ethylene-co-methacrylic acid),poly(ethylene-co-vinylacetate); the ionomeric ethylene copolymers;plasticized poly(vinyl chloride); and the aliphatic urethanes. Thesepolymers are preferred for one or more of the following reasons:suitable mechanical properties, good adhesion to the land layer orcube-corner elements, clarity, and environmental stability.

Selection of certain resins for the cube-corner elements and the bodylayer may result in an interpenetrating network after curing. Particularcombinations of resins for cube-corner elements and body layer can bereadily screened for penetration by application of a quantity of thecube-corner resin to the body layer. Priola, A., Gozzelino, G., andFerrero, F., Proceedings of the XIII International Conference in OrganicCoatings Science and Technology, Athens, Greece, Jul. 7-11, 1987, pp.308-18, discloses a watch glass test suitable for this purpose. See alsoU.S. patent application Ser. No. 07/472,444 filed Jun. 7, 1995.

In an embodiment that contains polycarbonate cube-corner elements and/ora polycarbonate land layer and a body layer that contains a polyethylenecopolymer such as poly(ethylene-co-(meth)acrylic acid),poly(ethylene-co-vinylacetate) or poly(ethylene-co-acrylate), theinterfacial adhesion between the body layer and the land layer orcube-corner elements can be improved by placing a thin tie-layer (notshown) therebetween. The tie-layer can be applied on the body layerbefore laminating the body layer to the land layer or to the cube-cornerelements. The tie-layer can be applied as a thin coating using, forexample: an aliphatic polyurethane in organic solution, for examplePermuthane™ U26-248 solution, available from Permuthane Company,Peabody, Mass.; Q-thane™ QC4820 available from K. J. Quinn and Co.,Inc., Seabrook, N.H.; an aliphatic polyurethane waterborne dispersion,for example NeoRez™ R-940, R-9409, R-960, R-962, R-967, and R-972,available from ICI Resins US, Wilmington, Mass.; an acrylic polymerwater borne dispersion, for example, NeoCryl™ A-601, A-612, A-614,A-621, and A-6092, available from ICI Resins US, Wilmington, Mass.; oran alkyl acrylate and aliphatic urethane copolymer water bornedispersion, for example NeoPac™ R-9000, available from ICI Resins US,Wilmington, Mass. In addition, an electrical discharge method, such as acorona or plasma treatment, can be used to further improve the adhesionof tie-layer to the body layer or the tie-layer to the land layer or tothe cube-corner elements.

Cube-corner retroreflective sheetings that are produced in accordancewith the second technique may be made from polymers discussed above asbeing applicable in the first technique. That is, the cube-cornerelements may comprise harder, or high modulus polymer(s) and the bodyportion may comprise softer, or lower modulus polymer(s). In addition tothese materials, cube-corner sheetings that comprise harder body layerpolymers such as polyesters or polycarbonates may also be made by thesecond technique. Further, when sheeting is made by the second techniquethe chemistry applicable to the cube-corner elements is broader than inthe first technique, that is, cube-corner elements may comprise eitherhard or soft polymers. U.S. patent application Ser. No. 08/625,857 toWilson et al. (filed Apr. 1, 1996) discloses examples of polymers thatmay be used in the cube-corner elements of the present invention.

When an article of the invention is prepared in accordance with thesecond technique, soft polymers--that is, polymers having an elasticmodulus less than 10×10⁸ pascals--may be used to produce the cube-cornerelements in glittering retroreflective sheeting. In the secondtechnique, the cube-corner elements are not subjected to the heat and/orpressure conditions of the batchwise or continuous processes of thefirst technique because the cube-corner element orientations aredetermined by the configuration of the mold. That is, glitteringsheetings made by the second technique receive cube-corner elementorientations directly from the mold. Distortion of the cube-cornerelements therefore is much less a concern, and it is possible to produceglittering sheetings that comprise only, or consist essentially of, softpolymers throughout the construction.

Example of soft polymers that can be used to make glittering cube-cornersheeting using the second technique include flexible poly(vinyl halides)such as poly(vinyl chloride), poly(vinylidene chloride); PVC-ABS;reactive and nonreactive vinyl resins; vinyl acrylates; mixtures ofvinyl acrylates with acrylated epoxies; polysiloxanes;allylalkoxysilanes; acrylated polysiloxanes; polyurethanes; acrylatedurethanes; polyesters; acrylated polyesters; polyethers; acrylatedpolyethers; acrylated oils; poly(tetrafluoroethylene);poly(fluoroethylene-co-fluoropropylene);poly(ethylene-co-tetrafluoroethylene); polybutylene; polybutadiene;poly(methylpentene); polyethylenes such as low density, high density,and linear low density; poly(ethylene-co-vinyl acetate); poly(ethylene-ethyl acrylate).

These polymers can be used either alone or may be blended together.Further, they can be blended with those described for the firsttechnique to give glittering cube-corner retroreflective sheeting viathe second technique. In addition, adjusting the crosslink density ofthe reactive polymers or blends listed for the first technique can alsoyield soft materials. The properties of the nonreactive polymers can beadjusted by changing the concentration of additives such as plasticizer,or by selection of different polymer grades.

Colorants, UV absorbers, light stabilizers, free radical scavengers orantioxidants, processing aids such as antiblocking agents, releasingagents, lubricants, and other additives may be added to the body portionor cube-corner elements. The particular colorant selected, of course,depends on the desired color of the sheeting. Colorants typically areadded at about 0.01 to 0.5 weight percent. UV absorbers typically areadded at about 0.5 to 2.0 weight percent. Examples of UV absorbersinclude derivatives of benzotriazole such as Tinuvin™ 327, 328, 900,1130, Tinuvin-P™, available from Ciba-Geigy Corporation, Ardsley, N.Y.;chemical derivatives of benzophenone such as Uvinul™-M40, 408, D-50,available from BASF Corporation, Clifton, N.J., or Cyasorb™ UV531 fromCytech Industries, West Patterson, N.J.; Syntase™ 230, 800, 1200available from Neville-Synthese Organics, Inc., Pittsburgh, Pa.; orchemical derivatives of diphenylacrylate such as Uvinul™-N35, 539, alsoavailable from BASF Corporation of Clifton, N.J. Light stabilizers thatmay be used include hindered amines, which are typically used at about0.5 to 2.0 weight percent. Examples of hindered amine light stabilizersinclude Tinuvin™-144, 292, 622, 770, and Chimassorb™-944 all availablefrom the Ciba-Geigy Corp., Ardsley, N.Y. Free radical scavengers orantioxidants may be used, typically, at about 0.01 to 0.5 weightpercent. Suitable antioxidants include hindered phenolic resins such asIrganox™-1010, 1076, 1035, or MD-1024, or Irgafos™168, available fromthe Ciba-Geigy Corp., Ardsley, N.Y. Small amount of other processingaids, typically no more than one weight percent of the polymer resins,may be added to improve the resin's processibility. Useful processingaids include fatty acid esters, or fatty acid amides available fromGlyco Inc., Norwalk, Conn., metallic stearates available from HenkelCorp., Hoboken, N.J., or Wax E™ available from Hoechst CelaneseCorporation, Somerville, N.J. Flame retardants--such as TetrabromoBisphenol A Diacrylate Monomer, SR 640, from Sauromer Company, Inc.,Exton, Pa., or Tricresyl phosphate, Kronitex™ TCP, from FMC Corporation,Philadelphia, Pa.--also may be added to the polymeric materials of theinventive sheeting to optimize its overall properties, as well as theproperties of the article to which it may be attached.

Flexible glittering retroreflective sheeting may be used on irregularsurfaces such as corrugated metal. For example, the sheeting may beplaced over the sidewall of a truck trailer or on a flexible surfacesuch as an article of clothing. Other applications for such glitteringretroreflective sheeting include warning flags, road signs, trafficcones, light wands, and vehicle conspicuity markings. When used on lightwands, the sheeting may be placed in a tubular configuration. Forexample, the sheeting can be adapted in the form of a tube or cylinder,and a light source may be directed into the tubular glittering article.The tubular glittering sheeting may be adapted with a fitting thatallows it to be secured to a light source such as at the end of aflashlight. Glittering retroreflective sheetings also may be embossed orotherwise adapted into three dimensional structures as taught in U.S.patent application Ser. No. 08/641,126 entitled "Formed Ultra-FlexibleRetroreflective Cube-Corner Composite Sheeting with Target OpticalProperties and Method for Making Same" (attorney docket number52477USA3A) filed on the same day as this patent application.

The invention is further illustrated in detail by the followingExamples. While the Examples serve this purpose, it should be understoodthat the particular ingredients used as well as other conditions anddetails are not to be construed in a manner that would unduly limit theinvention.

EXAMPLES

Retroeflective Brightness Test

The coefficient of retroreflection, R_(A) was measured in accordancewith standardized test ASTM E 810-93b. R_(A) values are expressed incandelas per lux per square meter (cd·lx⁻¹ ·m⁻²). The entrance angleused in ASTM E 810-93b was -4 degrees, and the observation angle was 0.2degrees. Further reference to "ASTM E 810-93b" means ASTM E 810-93bwhere the entrance and observation angles are as specified in theprevious sentence.

Lightness Test

Lightness of the cube-corner sheeting was measured using aspectrocolorimeter according to standardized test ASTM E 1349-90 .Lightness is expressed by the parameter termed Luminance Factor Y (LFY),which is defined as the lightness of the test sample relative to aperfect diffusing reflector. Zero degree illumination and 45 degreecircumferential viewing were employed in determining the LFY. LFY valuesrange from 0 to 100, where a LFY value of 0 represents black and a LFYvalue of 100 represents white.

Examples 1a-1ee

Batchwise Production of Glittering Article

Ordered retroreflective cube-corner sheeting prepared as described inExample 1 of U.S. patent application Ser. No. 08/472,444 filed Jun. 7,1995, was used. The sheeting included cube-corner retroreflectiveelements that measured approximately 0.0035 inches (90 micrometers (μm))from apex to base, and made from 1,6-hexanediol diacrylate,trimethylolpropane triacrylate, and bisphenol A epoxy diacrylate, in aratio of 25:50:25 parts by weight with 1% resin weight Darocur™4265 asphotoinitiator, a 0.01 inch (250 μm) thick clear, colorless, flexiblevinyl body layer, and a polyethylene terephthalate carrier film 0.002inches (50 μm) thick. The resin was cured through the film with a FUSIONH lamp (available from Fusion UV Curing Systems, Gaithersburg, Md.)operating at 235 Watt/cm at 25 ft/min (7.6 m/min), and then postcuredfrom the back side of the cube-corner elements with an AETEK mediumpressure mercury lamp (available from AETEK International, Plainfield,Ill.) operating at 120 Watt/cm at 25 ft/min (7.6 m/min). The sheetingwas placed onto Kraft release paper (Scotchcal™SCW 98 Marking Film, 3M,Saint Paul, Minn.) with the exposed cube-corner elements pointingdownward toward the paper. Together the Kraft paper and orderedcube-corner sheeting were placed onto the rubber surface of a Hix ModelN-800 heat lamination machine (Hix Corporation, Pittsburg, Kans.) thatwas preheated to 350° F. (175° C.), with the Kraft paper resting on therubber surface. The lamination machine was adjusted to apply 40 psi(2.75×10⁵ pascals (Pa)) air line pressure at 350° F. (175° C.) for 45seconds. The lamination machine was activated and, at the end of theheating period, the cube-corner sheeting was removed. After cooling toroom temperature, the polyester film was removed from the body layer toreveal cube-corner retroreflective sheeting capable of glittering. Otherprocessing conditions were used to prepare glittering retroreflectivesheeting where the temperature, time, and pressure were changed. Theeffects of these changes on glittering and retroreflective brightnessare illustrated in Table 1.

Example 1s was tested for lightness, and this sample exhibited an LFYvalue of 37.73.

                  TABLE 1                                                         ______________________________________                                        Effect of Batchwise Process                                                   Conditions on Formation of Glittering Sheeting                                                           Mean                                                                          Brightness                                                                    R.sub.A at                                               Temp   Time    Pressure                                                                            (cd/lux/m.sup.2)                                   Entry (°F.)                                                                         (sec)   (psi) 0°                                                                            90°                                                                         Comments                               ______________________________________                                        1a    195    45      40    528    440  No glittering                          1b    225    45      40    599    514  No glittering                          1c    249    45      40    721    608  No glittering                          1d    275    45      40    1270   739  No glittering                          1e    300    45      40    919    834  No glittering                          1f    324    45      40    543    582  Slight glittering                      1g    340    45      40    303    302  Full glittering                        1h    349    45      40    253    268  Full glittering                        1i    374    45      40    197    238  Full glittering                        1j    401    45      40    105    137  Full glittering                        1k    350    60      40    254    222  Full glittering                        1l    350    40      40    234    222  Full glittering                        1m    350    30      40    342    356  Full glittering                        1n    350    20      40    482    502  Full glittering                        1o    350    18      40    624    602  Full glittering                        1p    350    16      40    670    670  Full glittering                        1q    350    14      40    580    658  Full glittering                        1r    350    12      40    655    743  Full glittering                        1s    350    10      40    1086   874  Medium glittering                      1t    350    8       40    1357   860  Medium glittering                      1u    350    6       40    1136   847  Slight glittering                      1v    350    4       40    1245   789  No glittering                          1w    350    2       40    845    727  No glittering                          1x    350    10      5     --     --   Slight glittering                      1y    350    10      10    --     --   Medium glittering                      1z    350    10      20    --     --   Full glittering                        1aa   350    10      30    --     --   Full glittering                        1bb   350    10      40    --     --   Full glittering                        1cc   350    10      50    --     --   Full glittering                        1dd   350    10      60    --     --   Full glittering                        1ee   350    10      70    --     --   Full glittering                        ______________________________________                                    

Examples 2a-2m

Imaged Glittering Article Formed Using Flexographic Printing Plate

Ordered cube-corner retroreflective sheeting, as described in Examples1a-1ee, was used. A sheet of Kraft release paper was placed on therubber mat of a Hix Model N-800 heat lamination machine. On top of thepaper sheet was placed a flexographic printing plate having a raisedimage (FIG. 16a) in the shape of the letters "JPJ" surrounded by acircle. Ordered retroreflective cube-corner sheeting having a polyestercarrier on top of the body layer was placed onto the flexographicprinting plate such that the backside of the cube-corner elementscontacted the projecting image elements of the printing plate. A secondpiece of Kraft release paper was placed on top of the cube-cornersheeting. This arrangement corresponds to FIG. 11 where the flexographicplate is represented by 104. The assembly was heated to 350° F. (175°C.) with an air line pressure (psi) and for the times listed below inTable 2. When the lamination cycle ended, the lamination machine wasopened and the retroreflective cube-corner sheeting was removed. Whenthe sheeting cooled to room temperature, the optional polyester film (ifused) was removed to reveal a cube-corner retroreflective sheetingcapable of glittering. Several types of "JPJ" image were prepareddepending on construction and processing conditions and these areoutlined below in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Glittering Images Formed by Various Processing                                Conditions and Configurations at 350° F. (175° C.)                   Optional                                                                      Polyester                                                                          Flexographic                                                                              Air Line                                                                           Time                                               Example                                                                            Carrier                                                                            Plate Position                                                                            Pressure                                                                           (sec)                                                                            Image Description                               __________________________________________________________________________    2a   Absent                                                                             Contacting cube-corners                                                                   50   45 Nearly die cut image with piled cubes                                         adjacent.                                                                     Glittering on and beside image and in                                         background.                                     2b   Absent                                                                             Contacting cube-corners                                                                   20   3  No die cutting. Glittering image with                                         glittering                                                                    in background, none between images              2c   Absent                                                                             Contacting body layer                                                                     20   20 Very faint image, very faint glittering.                                      Background and area between images not                                        glittering.                                     2d   Absent                                                                             Contacting body layer                                                                     30   20 Very faint image, very faint glittering.                                      No                                                                            glittering in background or between                                           images.                                         2e   Absent                                                                             Contacting body tayer                                                                     50   20 Faint image, faint glittering in image                                        shape,                                                                        more than Examples 2c and 2d. No glittering                                   in                                                                            background or between images.                   2f   Absent                                                                             Contacting body layer                                                                     50   45 Full image with complete glittering only in                                   image                                                                         shape. No glittering in background or                                         between                                                                       images.                                         2g   Present                                                                            Contacting cube-corners                                                                   40   20 Very strong image shape, nearly die cut.                                      Little                                                                        glittering in image shape; strong                                             glittering                                                                    adjacent to image shape, in background,                                       some                                                                          between images.                                 2h   Present                                                                            Contacting cube-corners                                                                   40   6  Strong image; much less die cutting than                                      2g.                                                                           Glittering in image shape, background, and                                    between images.                                 2i   Present                                                                            Contacting cube-corners                                                                   10   20 Very strong image; less die cutting than                                      Example                                                                       2h. Glittering in image shape, background,                                    and                                                                           some between images. Piled glittering                                         cube-                                                                         corners.                                        2j   Present                                                                            Contacting cube-corners                                                                   10   3  Strong image, glittering in image shape, no                                   piled                                                                         cube-corners. Much less glittering in                                         background and between images than Example                                    2i.                                             2k   Present                                                                            Contacting cube-corners                                                                   20   3  Strong image, glittering in shape of image,                                   no                                                                            piled cube-corners. More glittering in                                        background and between images than Example                                    2j.                                             2l   Present                                                                            Contacting cube-corners                                                                   20   6  Strong image, glittering in shape of image,                                   no                                                                            piled cube-corners. More background and                                       between image glittering than Example 2k,                                     much                                                                          more than Example 2j.                           2m   Present                                                                            Contacting polyester carrier                                                              40   20 Image extremely faint and not fully formed.                                   No                                                                            glittering anywhere.                            __________________________________________________________________________

Examples 3a-3f

Creation of Images Using a Polyester Film

Ordered retroreflective cube-corner sheeting and the apparatus describedin Examples 1a-1ee were used in the absence of the lower release paper76. Polyester film of several thicknesses was used as the image formingelement 104 and was positioned so that it touched the back side of thecube-corner elements. To make positive images on glittering sheeting,the shape of the geometric figures of a square, a circle, and a triangle(each approximately 0.5 inch (1.25 cm) in outside dimension) were cutfrom a 4×6 inch sheet of polyester film of known thickness. Theresulting polyester film image forming element was positioned as 104 asshown in FIG. 11. To make negative images on glittering, texturedsheeting, the geometric figures that had been cut out to make thepositive image forming element were placed directly on the unheatedlaminator surface 74. The images were formed in the glittering texturedcube-corner retroreflective sheeting by operating the heat laminationmachine at 350° F. for 45 seconds at a line pressure of 40 psi (2.75×10⁵Pa). Descriptions of the imaged sheetings are listed in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Formation of Images with Polyester Film Image Forming Elements                           Thickness of Polyester                                                        Image Forming                                                      Example                                                                            Image Type                                                                          Element (μm)                                                                             Description of Image and Background                  __________________________________________________________________________    3a   Positive                                                                            180       Very strong textured and glittering effect in the                             shape of the                                                                  cut out geometric figures on a slightly glistening                            background.                                                                   Both image and background are retroreflective.                                Image                                                                         exhibits rainbow effect, background does not.            3b   Negative                                                                            180       Slightly glistening to unchanged image of the                                 geometric                                                                     figures on a strongly textured and glittering                                 background.                                                                   Both image and background are retroreflective; only                           background exhibits rainbow effect.                      3c   Positive                                                                            100       Very strong textured, glittering image in the shape                           of the                                                                        geometric figures on a glistening background. Image                           and                                                                           background are retroreflective; only glittering,                              textured image                                                                exhibits rainbow effect.                                 3d   Negative                                                                            100       Slightly glistening image in shape of geometric                               shapes on a                                                                   strongly textured and glittering background. Image                            and                                                                           background are both retroreflective; only                                     glittering, textured                                                          background exhibits rainbow effect.                      3e   Positive                                                                            50        Strongly textured and glittering image in the shape                           of the                                                                        geometric figures on a glistening background with                             large                                                                         irregular portions of the background exhibiting                               glittering and                                                                rainbow effect. Background and images were both                               retroreflective.                                         3f   Negative                                                                            50        Glistening image in the shape of the geometric                                figures on a                                                                  strongly textured and glittering background. Image                            and                                                                           background are both retroreflective; only textured,                           glittering                                                                    background exhibits rainbow effect.                      __________________________________________________________________________

Example 4

Creation of Image Using Transfer Ink

Ordered retroreflective cube-corner sheeting and the apparatus describedin Examples 1a-1ee were used with optional polyester carrier in place.The image forming element was a piece of black printed label tape (FIG.16b) made with a Merlin Express Elite label tape machine (VaritronicSystems, Inc., Minneapolis, Minn.) and was positioned with the ink imagetouching the cube-corner elements. The lamination machine was keptclosed at 350° F. (175° C.) for 45 seconds at 40 psi (2.75×10⁵ Pa) airline pressure. At the end of the processing cycle, the sheeting wasremoved from the machine. When it returned to room temperature, thepolyester carrier was removed to reveal a glittering cube-cornerretroreflecting sheet material with a black ink image transferred fromthe label tape. Examination of the sheeting with retroreflectedillumination revealed a retroreflectively dark image on a glittering andretroreflective background.

Example 5

Glittering Image Produced from Woven Fabric

Ordered retroreflective cube-corner sheeting and apparatus described inExamples 1a-1ee were used, with the optional polyester carrier in place.The image forming element was a piece of polyester plain weave fabric2.2 oz/yd² (188 g/m²) and was located as illustrated by 104 in FIG. 11.The processing cycle was allowed to continue for 45 seconds at 350° F.(175° C.) with 40 psi (2.75×10⁵ Pa) line pressure. After the sheetingwas cooled to room temperature, the polyester carrier was removed toreveal a cube-corner retroreflective sheeting that contained an overalltexture in the pattern of the fabric used and with glittering effect inaddition to the overall texture. The glittering textured sheeting thusprepared displayed more intense glittering than sheeting prepared byExample 1 with no texture.

Example 6

Glittering Sheeting Produced by a Continuous Process

Ordered retroreflective cube-corner sheeting as described in Examples1a-1ee was passed through a continuous nip type lamination station asillustrated in FIG. 12. The apparatus was custom built and comprised aheated stainless steel roll 77, an unheated rubber coated roll 77', amechanism for controlling and adjusting, by air pressure, the force withwhich the nip of heated roll 77 and unheated roll 77' meet, and a meansof controlling the speed at which the drive roll moves. The continuouslamination apparatus was adjusted to a speed of 1.5 ft/min (30.5cm/min), 375° F. (175° C.) heated roll, 40 psi (2.75×10⁵ Pa) nip closurepressure. Sheets of the ordered cube-corner retroreflective sheeting 3inches by 17 inches (7.5×43 cm) were fed into the moving nip withcube-corner elements touching the unheated rubber coated roll. Thesheets were collected after passing through the nip, cooled to roomtemperature, and the polyester carrier was removed to give a glitteringcube-corner retroreflective sheeting. Other processing conditions wereused to prepare glittering retroreflective sheeting where thetemperature, speed, and nip pressure were changed. Changing theseconditions had similar effects on the glittering retroreflectivesheetings as observed by changing process conditions in the batchwiseprocess described in Example 1. Similar results were achieved using acontinuous roll of sheeting.

Example 7

Glittering Sheeting Produced from an Electroformed Mold

Cube-corner retroreflective sheeting capable of glittering prepared asdescribed in Example 1h was positioned on a backing support and fixed inplace with double sided adhesive tape. A silver metal coating wasprovided over the entire surface by electroless deposition for renderingthe glittering cube-corner retroreflecting sheeting conductive forelectroplating. The resulting assembly was immersed in a nickelsulfamate bath containing 16 ounces/gallon (120 g/L) of nickel; 0.5ounces/gallon (3.7 g/L) of nickel bromide; and 4.0 ounces/gallon (30g/L) of boric acid. The remainder of the plating bath was filled withdistilled water. A quantity of S-nickel anode pellets were containedwithin a titanium basket that was suspended in the plating bath. A wovenpolypropylene bag was provided that surrounded the titanium basketwithin the plating bath for trapping particulates. The plating bath wascontinuously filtered through a 5 micrometer filter. The temperature ofthe bath was maintained at 90° F. (32° C.) and a pH of 4.0 wasmaintained in the plating bath solution. A current density of 20 ampsper square foot (215 amp/square meter) was applied to the system for 24hours with the mounted sheeting continuously rotated at 6 rpm to enhancea uniform deposit. Upon removal from the electroforming bath, thecube-corner retroreflective sheeting capable of exhibiting theglittering effect was removed from the electrodeposited metal to give anickel mold, approximately 0.025 inches (approximately 0.063 cm) thick,which was the negative image of the original glittering cube-cornerretroreflective sheeting. The mold alone displayed the properties ofglittering, although it did not exhibit the rainbow hues of which thesheeting was capable, and the mold was retroreflective.

A mixture of 1,6-hexanediol diacrylate, trimethylolpropane triacrylate,and bisphenol A epoxy diacrylate in a ratio of 25:50:25 parts by weightwith 1% resin weight of Darocur™ 4265 as photoinitiator (Radcure IRR1010, Lot N215-0302, UCB Radcure, Smyrna, Ga.) was carefully applied toone edge of the electroformed mold. The bank of resin was slowly rolledacross the mold allowing the resin to fill all features of the mold.When a smooth coating of resin was in the mold it was covered by rollingon a vinyl film sheet, 0.010 inches (0.025 cm) thick (American RenolitCorporation, Whippany, N.J. 07981). The resulting constructioncontaining wet resin was cured through the vinyl film by passage througha Fusion Model DRS-120QN system and exposure to a FUSION V lampoperating at high power (235 Watts/cm) at a rate of 25 ft/min. (7.6n/sec). Removal of the cured sheeting from the mold gave a sheeting thatwas post cured on the backside of the cube-corner element array bypassage under a FUSION H lamp at 25 ft./min. (7.6 m/sec) at high power(235 Watts/cm). The resulting cube-corner sheeting, made from theelectroformed mold was retroreflective, glittered, and exhibited arainbow of colors in the points of light.

Example 8

Glittering Sheeting Produced from an Electroformed Mold with an InkImage

An image in the shape of "3M" was made with nonaqueous stamp pad ink onthe cube-corner side of cube-corner retroreflective sheeting that wasprepared as described in Example 1h. When the ink had dried theresulting glittering cube-corner retroreflective sheeting bearing theink image was mounted, prepared, and electroformed as described inExample 7. Removal of the sheeting from the electroformed mold gave anickel mold, approximately 0.025 inches (approximately 0.063 cm) thick,which bore a reverse image of the rubber stamp. This mold was used toprepare cube-corner sheeting in accordance with Example 7. After curingand removing the newly formed sheeting from the mold, the sheeting wasobserved to be retroreflective, capable of exhibiting the glitteringeffect, capable of exhibiting the rainbow effect, and the sheeting borethe image of "3M" as stamped on the original sheeting from which themold was made. The image appeared on the sheeting as anonretroreflective glittering image on a retroreflective background.

Examples 9a-9f

Screen Printed Images

A screen printing hand table (Model 1218 AWT World Trade, Inc., Chicago,Ill.) was fitted with a 110 T (mesh/inch) printing screen bearing theimage "Atlanta 1996". Ordered cube-corner retroreflective sheeting asdescribed in Examples 1a-1ee was placed on the printing surface andprinted on with the GV-159 transparent permanent blue ink (Naz-DarCorporation, Chicago, Ill. 60622-4292), or with SX 863 transparent greenink (Plast-O-Meric SP, Inc., Sussex, Wis. 53089-0375), or with SX 864 Bopaque purple ink (Plast-O-Meric). When the cube-corner elements werefacing upwards during printing, the screen printed image was formed onthe back side of the cube-corner elements. When the cube-corner elementswere facing downwards during printing, the screen printed image wasformed on the front, vinyl film surface of the cube-corner sheeting.Sheeting with images printed with GV-159 Permanent Blue was air driedover night before further processing. Sheeting with images printed withSX 863 or with SX 864 B was gelled with a Texair Model 30 screenprinting belt oven (American Screen Printing Equipment Company, Chicago,Ill. 60622) adjusted so that the infrared panel would operate at 1100°F. (593° C.), the electrically heated forced air was in the "off"position, and a belt speed to allow residence time of 42-46 seconds,before further processing. After initial drying or gelation, the screenprinted cube-corner sheetings were processed under heat and pressure asdescribed in Example 1. Results of the processing are listed below inTable 4.

                                      TABLE 4                                     __________________________________________________________________________    Screen Printed Images on Cube-Corner                                          Retroreflective Sheeting Capable of the Glittering Effect                     Example                                                                            Ink  Formula   Image Sheet Side                                                                          Description                                   __________________________________________________________________________    9a   GV-159                                                                             Used as Supplied                                                                        Transparent                                                                         Vinyl Blue retroreflective through image.                                           Glitter blue from both                             Permanent            Front sides through image. Full glitter outside                                     image area.                                        Blue                                                                     9b   GV-159                                                                             Used as Supplied                                                                        Transparent                                                                         Cube-Corner                                                                         No retroreflection in image area.                  Permanent            Back  Subdued glitter in image on both back and                                     front. Full                                        Blue                       glitter outside image.                        9c   SX 863                                                                             Cyan (8 parts)                                                                          Transparent                                                                         Vinyl Green retroreflective through image.                                          Glitter green through                              Green                                                                              Yellow (1 part) Front image from both sides. Full glitter                                           outside image area.                           9d   SX 863                                                                             Cyan (8 parts)                                                                          Transparent                                                                         Cube-Corner                                                                         Not retroreflective through image. No                                         glitter in image                                   Green                                                                              Yellow (1 part) Back  area from either side. Full glitter                                           outside image area from                                                       both sides. Extreme clarity through                                           image.                                        9e   SX 864 B                                                                           Cyan (10.6 parts)                                                                       Opaque                                                                              Vinyl No retroreflection in image area. No                                          glittering from front                              Blue Magenta (17.7 parts)                                                                          Front side in image. Strong white glittering                                        from back side in                                       White (4.3 parts)     image. Full glitter outside image area                                        from both sides.                              9f   SX 864 B                                                                           Cyan (10.6 parts)                                                                       Opaque                                                                              Cube-Corner                                                                         No retroreflection in image area.                  Blue Magenta (17.7 parts)                                                                          Back  No glittering in image from either side.                                      Full glitter outside                                    White (4.3 parts)     image area on both sides.                     __________________________________________________________________________

Examples 10a-10n

Vapor Coated Sheeting

Ordered nonrandom retroreflective cube-corner sheeting and the apparatusused in Examples 1a-1ee above were used. Retroreflective sheeting wasprepared with a vapor deposited layer of material approximately 850 Åthick. The ordered cube-corner retroreflective sheeting was installed ina bell jar type vacuum apparatus having an approximate capacity of 250liters(Model 900-217-12, Stokes Vacuum Equipment, Equipment Division ofPennsalt Chemical Corporation, Philadelphia, Pa. 19120). Afterevacuation of the bell jar to 10⁻⁵ Torr or less, the material intendedto be vacuum deposited on the sheeting was irradiated with an electronbeam (Airco Temescal, Electron Beam Power Supply Model CV-10, Berkeley,Calif.) until deposition on the cube side of the sheeting was complete.The resulting vapor coated, ordered nonrandom cube-corner sheeting wasprocessed by heat and pressure as described in Example 1 to givecube-corner sheeting capable of displaying very strong, extremelybrilliant glittering from both sides. The sheeting prepared in thismanner appeared to have better lightness than the sheeting that is vaporcoated but does not have the cube-corner elements oriented in accordancewith the invention. Table 5 below lists representative materials thathave been vapor coated onto ordered, nonrandom cube-corner sheeting.After vapor coating, all the sheetings were processed by heat andpressure to make a sheeting capable of glittering. Table 5 also shows abrief characterization of the vapor coated sheetings.

The two steps of this example, vapor coating, then processing by heatand pressure, may be accomplished in reverse order with the sameoutcome. That is, ordered, nonrandom cube-corner sheeting may first beprocessed as described in Example 1 to provide sheeting capable ofexhibiting the glittering effect. The resulting glittering sheeting maythen be subjected to vacuum deposition of materials on the cube-cornerside to give cube-corner sheeting that is capable of exhibiting verystrong, extremely brilliant glittering from both sides. The columnheading "Processing Sequence" in Table 5 refers to whether thecube-corner sheeting was made glittering first and then was vapor coatedor was vapor coated and then made glittering. The listing "Glittering,then VC" refers to sheeting that was made glittering in a firstoperation then vapor coated in a second operation. The listing "VC thenglitter and texture" refers to sheeting that was vapor coated in a firstoperation then made glittering in a second operation. In this case, thevapor coated sheeting was made glittering in the absence of the lowerrelease paper 76 in FIG. 11 and the resulting sheeting has theglittering effect superimposed on an overall pattern or texture from thelower, unheated rubber platen 74.

Examples 10a and 10b were tested for lightness, and these samplesexhibited LFY values of 16.7 and 18.9, respectively.

                                      TABLE 5                                     __________________________________________________________________________    Glittering Cube-Corner Sheetings Prepared by Vacuum Deposition and Heat       and Pressure Processing                                                                            Brightness,                                                                   R.sub.A                                                  Processing    Vapor Coated                                                                         (cd/lux/m.sup.2)                                         Example                                                                            Sequence Material                                                                             0°                                                                        90°                                                                       Description                                        __________________________________________________________________________    10a  Glittering, then VC                                                                    Aluminum                                                                             384                                                                              791                                                                              Silver gray withstrong glitter and rainbow                                    from                                                                          front, strong white glitter from back side.        10b  VC then glitter and                                                                    Aluminum                                                                             240   Vapor coated standard sheeting became fully             texture               glittering after processing with some rainbow                                 from front. Strong white glitter from back                                    side.                                              10c  Glittering, then VC                                                                    Copper 303                                                                              301                                                                              Beautiful red-bronze color with full glitter                                  and                                                                           rainbow shifted toward shades of red. With                                    retro-illumination sheeting exhibits                                          glittering and                                                                retroreflection separately, both in color.         10d  VC then glitter and                                                                    Copper 72    Vapor coated standard sheeting became fully             texture               glittering after processing with rainbow from                                 front side. Back is copper colored with                                       strong                                                                        glittering; larger and stronger when                                          transmitted                                                                   from opposite side than when reflected.            10e  Glittering, then VC                                                                    ZnS    355                                                                              352                                                                              Retroreflective and fully glittering. Rainbow                                 from both sides.                                   10f  VC then glitter and                                                                    ZnS    154   Retroreflective with full glittering and                                      rainbow                                                 texture               from both sides; larger and stronger when                                     transmitted from opposite side than when                                      reflected from both side.                          10g  Glittering, then VC                                                                    ZnS/Cryolite                                                                         344                                                                              506                                                                              Retroreflecting and fully glittering. Rainbow                                 from both sides.                                   10h  VC then glitter and                                                                    ZnS/Cryolite                                                                         110   Retroreflecting with full glittering and                                      rainbow                                                 texture               from both sides; larger and stronger when                                     transmitted from opposite side than reflected                                 from same side.                                    10i  Glittering, then VC                                                                    SiO    558                                                                              884                                                                              Retroreflective, transparent sheeting with                                    full                                                                    224   glittering and rainbow from both sides.            10j  VC then glitter and                                                                    SiO          Retroreflective with full glittering and                                      rainbow                                                 texture               from both sides; stronger when transmitted                                    from                                                                          opposite side than when reflected from same                                   side.                                              10k  Glittering, then VC                                                                    ZnS/Al 54 59 Opaque and dull gray with full glittering and                                 some rainbow. Back side is bright silver and                                  glittering.                                        10l  VC then glitter and                                                                    ZnS/Al 37    Opaque gray with fine grained glitter and                                     small                                                   texture               rainbow. Back side is brilliantly silver and                                  glittering.                                        10m  Glittering, then VC                                                                    20% TiO.sub.2                                                                        128                                                                              107                                                                              Dull gray-brown, poorly transparent and                          80% Bi.sub.2 O.sub.3                                                                       retroreflective. Full glittering and rainbow                                  have                                                                          a metallic appearance because of brown                                        background.                                        10n  VC then glitter and                                                                    20% TiO.sub.2                                                                        28    Brown to golden front face with full glitter                                  and                                                     texture  80% Bi.sub.2 O.sub.3                                                                       rainbow; stronger when transmitted from other                                 side than reflected from same side. Back side                                 full glitter and rainbow in gold                   __________________________________________________________________________                               tones.                                         

Example 11

Preparation of Retroreflective Product Having Seal Film

Glittering cube-corner retroreflective sheetings prepared according toExample 9 were ultrasonically welded to a 0.01 inch (250 micrometers)thick white pigmented, embossed vinyl seal film (Nan Ya, Bachelor, La).The cube-corner elements of the screen printed glittering sheeting wereplaced in contact with the embossed side of the seal film, and a 0.002inch (50 micrometers) thick polyester film was placed on the unembossedside of the seal film. The construction was placed on a patterned anvilattached to the base of a Branson Model 184V ultrasonic welder with thepolyester sheeting facing the horn of the welder and the vinyl bodylayer of the glittering cube-corner sheeting touching the patternedanvil. The ultrasonic welder was operated at 20 Khz, 60 psi (4.2×10⁵Pa), 17 fpm (5.2 m/min), with an amplitude equal to 60% of maximum and a2.865 inch (7.277 cm) horn radius. The anvil comprised three 1 inch (2.5cm) wide lanes with adjacent triangles having sides approximately 1.5inches (3.5 cm) in length and bases approximately 2 inches (5 cm) inlength, and one 1 inch (2.5 cm) wide lane having diamonds with sidesapproximately 0.75 inches (2 cm) in length. The ultrasonic weldingprocess gave sealed samples whose seal lines were a clean reproductionof the anvil pattern.

All of the patents and patent applications cited above are whollyincorporated by reference into this patent application.

As illustrated by the above discussion, the invention may take onvarious modifications and alterations without departing from its totalscope and spirit. Accordingly, the invention is not limited to theabove-described but is to be controlled by the limitations set forth inthe claims and any equivalents thereof.

What is claimed is:
 1. A mold that comprises:an array of cube-cornerelements that are arranged in the array such that a retroreflectivecube-corner sheeting that is formed thereon is capable of glitteringwhen light is incident thereon, wherein the array of cube-cornerelements includes generally intersecting grooves, wherein at least onegroove has faces of adjacent cube-corner elements arranged such that adihedral angle α located between the adjacent faces varies along the atleast one groove.
 2. The mold of claim 1, wherein the cube-cornerelements each include a base plane, and the cube-corner elements arearranged such that the base planes do not all reside in the same generalplane when the sheeting is laid flat.
 3. The mold of claim 1, whereinthe cube-corner elements are randomly-tilted across at least a portionof the array.
 4. The mold of claim 1, wherein the cube-corner elementsin the array are arranged such that a sheeting formed thereon glittersfrom front and back sides of the sheeting when light strikes either thefront or the back.
 5. The mold of claim 1, wherein the cube-cornerelements have base edges that do not lie in the same common plane whenthe sheeting is laid flat.
 6. The mold of claim 1, wherein angle αvaries from 0 degrees to 180 degrees.
 7. The mold of claim 6, whereinangle α ranges from 35 to 115 degrees on average.
 8. The mold of claim1, wherein some cube-corner elements are piled up on each other.
 9. Themold of claim 1, wherein the array of cube-corner elements is arrangedto produce a glittering image on a non-glittering background.
 10. Themold of claim 1, wherein the array of cube-corner elements is arrangedto produce a non-glittering image on a glittering background.
 11. Themold of claim 1, wherein the array of cube-corner elements is arrangedto produce a glittering image on a glittering background, the imaged andbackground exhibiting varying degrees of glitter to render the imagedarea discernible from the background.
 12. The mold of claim 1, whereinthe cube-corner elements are arranged in the mold such that a sheetingformed thereon glitters to produce at least 10 points of light persquare centimeter when viewed under direct sunlight.
 13. The mold ofclaim 1, wherein the cube-corner elements are arranged in the mold suchthat a sheeting formed thereon glitters to produce at least 50 points oflight per square centimeter when viewed under direct sunlight.
 14. Themold of claim 12, wherein the cube-corner elements are arranged suchthat a sheeting formed thereon glitters to produce less than about 250points of light per square centimeter.
 15. The mold of claim 1 adaptedfor producing glittering retroreflective sheeting continuously.
 16. Themold of claim 14 being configured as an endless loop.
 17. The mold ofclaim 1 being made from a polymeric material(s) having a Shore Adurometer less than
 90. 18. The mold of claim 16, being made from apolymeric material(s) having a durometer less than Shore A
 60. 19. Amold that that comprises:(a) a base surface; and (b) a structuredsurface opposite the base surface, the structured surface comprising anarray of cube-corner elements, which array is defined by three sets ofintersecting grooves, wherein each groove set includes two or moregenerally parallel grooves, and at least one groove in at least one ofthe sets has faces of adjacent cube-corner elements arranged such that adihedral angle located between the adjacent faces varies along thegroove(s) in the set.
 20. A mold that comprises:a structured surfacethat includes a multiplicity of discreet cube-corner elements, thecube-corner elements each including a base surface and three faces, thecube-corner elements being arranged such that the base planes do notreside in the same plane when the sheeting is laid flat.
 21. A mold thatcomprises:(a) a base surface; and (b) a structured surface opposite thebase surface, wherein the structured surface comprises an array ofrandomly-tilted cube-corner elements.
 22. A combination that includesthe mold of claim 1 and a resin, the resin being disposed in the arrayof cube-corner elements for purposes of producing a glitteringcube-corner sheeting.
 23. A combination that includes the mold of claim19 and a resin, the resin being disposed in the array of cube-cornerelements for purposes of producing a glittering cube-corner sheeting.24. A combination that includes the mold of claim 21 and a resin, theresin being disposed in the array of cube-corner elements for purposesof producing a glittering cube-corner sheeting.
 25. A combination thatincludes the mold of claim 20 and a resin, the resin being disposed overthe multiplicity of cube-corner elements for purposes of producing aglittering cube-corner sheeting.
 26. The mold of claim 1 being disposedin an apparatus that is capable of producing glittering cube-cornersheeting continuously.
 27. The mold of claim 19 being disposed in anapparatus that is capable of producing glittering cube-corner sheetingcontinuously.
 28. The mold of claim 20 being disposed in an apparatusthat is capable of producing glittering cube-corner sheetingcontinuously.
 29. The mold of claim 21 being disposed in an apparatusthat is capable of producing glittering cube-corner sheetingcontinuously.
 30. The mold of claim 1, wherein the array of cube-cornerelements is defined by three sets of intersecting grooves, wherein eachgroove set includes two or more generally parallel grooves, and at leastone groove in at least one of the sets has faces of adjacent cube-cornerelements arranged such that a dihedral angle α located between theadjacent faces varies along the groove(s) in the set.
 31. The mold ofclaim 30, wherein at least one groove in each of the three sets ofintersecting grooves has faces of adjacent cube-corner elements arrangedsuch that the dihedral angle α located between the adjacent faces variesalong the grooves in all three groove sets.
 32. The mold of claim 31,wherein the cube-corner elements are about 60 to 200 micrometers highand exhibit a variation in height between adjacent apexes of 1 to 40micrometers on average.